emoji [nfc]: Compare emoji base ordering with web's, in detail

I spent some time reverse-engineering in full detail the behavior web
has for building its list of possible emoji to present in the UI (and
so for the order that emoji results have when they don't differ on
any of the criteria used at the ranking step after applying a query).

The first output of that study was discovering zulip#1113, fixed earlier
in this branch.  This records the rest.

lib/model/emoji.dart

@@ -222,6 +222,39 @@ class EmojiStoreImpl with EmojiStore {
   }
 
   List<EmojiCandidate> _generateAllCandidates() {
+    // Compare `emoji_picker.rebuild_catalog` in Zulip web;
+    // `composebox_typeahead.update_emoji_data` which receives its output;
+    // and `emoji.update_emojis` which builds part of its input.
+    //   https://github.com/zulip/zulip/blob/0f59e2e78/web/src/emoji_picker.ts#L132-L185
+    //   https://github.com/zulip/zulip/blob/0f59e2e78/web/src/composebox_typeahead.ts#L138-L163
+    //   https://github.com/zulip/zulip/blob/0f59e2e78/web/src/emoji.ts#L232-L278
+    //
+    // Behavior differences we might copy or change, TODO:
+    //  * Web has a particular ordering of Unicode emoji;
+    //    a data file groups them by category and orders within each of those,
+    //    and the code has a list of categories.
+    //    This seems useful; it'll call for expanding the server emoji data API.
+    //  * Both here and in web, the realm emoji appear in whatever order the
+    //    server returned them in; and that order appears to be random,
+    //    presumably the iteration order of some Python dict,
+    //    and to vary over time.
+    //
+    // Behavior differences that web should probably fix, TODO(web):
+    //  * Web ranks the realm's custom emoji (and the Zulip extra emoji) at the
+    //    end of the base list, as seen in the emoji picker on an empty query;
+    //    but then ranks them first, after only the six "popular" emoji,
+    //    once there's a non-empty query.
+    //  * Web gives the six "popular" emoji a set order amongst themselves,
+    //    like we do after #1112; but in web, this order appears only in the
+    //    emoji picker on an empty query, and is otherwise lost even when the
+    //    emoji are taken out of their home categories and shown instead
+    //    together at the front.
+    //
+    //    In web on an empty query, :+1: aka :like: comes first, and
+    //    :heart: aka :love: comes later (fourth); but then on the query "l",
+    //    the results begin with :love: and then :like:.  They've flipped order,
+    //    even though they're equally good prefix matches to the query.
+
     final results = <EmojiCandidate>[];
 
     final namesOverridden = {
@@ -252,8 +285,7 @@ class EmojiStoreImpl with EmojiStore {
     for (final emoji in activeRealmEmoji) {
       final emojiName = emoji.name;
       if (emojiName == 'zulip') {
-        // TODO does 'zulip' really override realm emoji?
-        //   (This is copied from zulip-mobile's behavior.)
+        // :zulip: overrides realm emoji; compare web's `emoji.update_emojis`.
         continue;
       }
       results.add(_emojiCandidateFor(