refactor: enforce single OAuth2 account binding per platform (#59)

### What this PR does?
限制每个平台的 OAuth2 帐户一次只能连接到一个用户帐户

/kind improvement

```release-note
限制每个平台的 OAuth2 帐户一次只能连接到一个用户帐户
```

src/main/java/run/halo/oauth/Oauth2Authenticator.java

@@ -1,5 +1,6 @@
 package run.halo.oauth;
 
+import static org.apache.commons.lang3.StringUtils.defaultIfBlank;
 import static org.apache.commons.lang3.StringUtils.defaultString;
 import static run.halo.oauth.SocialServerOauth2AuthorizationRequestResolver.SOCIAL_CONNECTION;
 
@@ -170,7 +171,7 @@ private ServerAuthenticationSuccessHandler registrationPageHandler(String regist
             Assert.notNull(oauth2User, "oauth2User cannot be null");
 
             String loginName = oauth2User.getName();
-            String name = defaultString(oauth2User.getAttribute("name"), loginName);
+            String name = defaultIfBlank(oauth2User.getAttribute("name"), loginName);
             MultiValueMap<String, String> queryParams = new LinkedMultiValueMap<>();
             queryParams.add("login", loginName);
             queryParams.add("name", name);

src/main/java/run/halo/oauth/OauthClientRegistrationRepository.java

@@ -43,13 +43,13 @@ public Mono<ClientRegistration> findByRegistrationId(String registrationId) {
                 Mono.error(new ProviderNotFoundException(
                     "Unsupported OAuth2 provider: " + registrationId)))
             .flatMap(provider -> fetchEnabledProviders()
-                .map(enabledNames -> {
-                    if (enabledNames.contains(registrationId)) {
-                        return provider;
+                .doOnNext(enabledNames -> {
+                    if (!enabledNames.contains(registrationId)) {
+                        throw new OAuth2AuthenticationException(
+                            "Authentication provider is not enabled: " + registrationId);
                     }
-                    throw new OAuth2AuthenticationException(
-                        "Authentication provider is not enabled: " + registrationId);
                 })
+                .thenReturn(provider)
             )
             .flatMap(this::getClientRegistrationMono);
     }

src/main/java/run/halo/oauth/UserConnectionServiceImpl.java

@@ -8,6 +8,7 @@
 import org.springframework.security.oauth2.core.user.OAuth2User;
 import org.springframework.stereotype.Component;
 import org.springframework.util.Assert;
+import org.springframework.web.server.ServerWebInputException;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import run.halo.app.core.extension.UserConnection;
@@ -39,14 +40,22 @@ public Mono<UserConnection> createConnection(String username,
 
         UserConnection connection = convert(username, authentication);
         String providerUserId = authentication.getPrincipal().getName();
-        return fetchUserConnection(connection.getSpec().getRegistrationId(), providerUserId)
-            .flatMap(persisted -> {
-                connection.getMetadata().setName(persisted.getMetadata().getName());
-                connection.getMetadata()
-                    .setVersion(persisted.getMetadata().getVersion());
-                return client.update(connection);
-            })
-            .switchIfEmpty(Mono.defer(() -> client.create(connection)));
+        return findByRegistrationId(connection.getSpec().getRegistrationId())
+            .hasElement()
+            .flatMap(exists -> {
+                if (exists) {
+                    return Mono.error(new ServerWebInputException(
+                        "已经绑定过 " + connection.getSpec().getRegistrationId() + " 账号，请先解绑"));
+                }
+                return fetchUserConnection(connection.getSpec().getRegistrationId(), providerUserId)
+                    .flatMap(persisted -> {
+                        connection.getMetadata().setName(persisted.getMetadata().getName());
+                        connection.getMetadata()
+                            .setVersion(persisted.getMetadata().getVersion());
+                        return client.update(connection);
+                    })
+                    .switchIfEmpty(Mono.defer(() -> client.create(connection)));
+            });
     }
 
     @Override
@@ -81,6 +90,12 @@ Flux<UserConnection> listByRegistrationIdAndUsername(String registrationId, Stri
             && persisted.getSpec().getUsername().equals(username), null);
     }
 
+    private Mono<UserConnection> findByRegistrationId(String registrationId) {
+        return client.list(UserConnection.class,
+                persisted -> persisted.getSpec().getRegistrationId().equals(registrationId), null)
+            .next();
+    }
+
     private Mono<UserConnection> fetchUserConnection(String registrationId, String providerUserId) {
         return client.list(UserConnection.class, persisted -> persisted.getSpec()
                 .getProviderUserId().equals(providerUserId)