Add the ability to configure multiple IdPs

[smbader]

This request is to modify the plugin so that multiple IdPs can be configured and used simultaneously. A new tab in the Shibboleth settings named "Identity Providers" will allow new providers to be added with the settings listed below.

Per-IdP settings:

• New field: IdP short code (A couple of letters so that we can provide direct links.)
• New field: entityId (The entityID is the public URI for an Identity Provider.)
• Password Change URL (If this option is set, Shibboleth users will see a "change password" link on their profile page directing them to this URL.)
• Password Reset URL (If this option is set, wp-login.php will send ALL users here to reset their password.)
• Button Text (Set the text of the button that appears on the wp-login.php page. This will be used to differentiate between IdPs)

Fixes #93

[jrchamp]

Thank you @smbader! This looks amazing.

I'm not sure why the CodeSniffer is trying to apply WordPress PHP rules to .js files. Maybe that's something they changed in a newer version of their eslint rules. I'll take a look into that.

Not to self - in case we should be running ESLint manually, it might look something like this:

    - name: Run ESLint
      run: yarn
      run: eslint . --ext .js

[jrchamp]

This is beautiful! I was able to fix the JavaScript spacing issues so the automated checks pass now.