Change admin UI for hashtags and add back whitelisted trends (#11490)

Fix #271

Add back the `GET /api/v1/trends` API with the caveat that it does
not return tags that have not been allowed to trend by the staff.

When a hashtag begins to trend (internally) and that hashtag has
not been previously reviewed by the staff, the staff is notified.

The new admin UI for hashtags allows filtering hashtags by where
they are used (e.g. in the profile directory), whether they have
been reviewed or are pending reviewal, they show by how many people
the hashtag is used in the directory, how many people used it
today, how many statuses with it have been created today, and it
allows fixing the name of the hashtag to make it more readable.

The disallowed hashtags feature has been reworked. It is now
controlled from the admin UI for hashtags instead of from
the file `config/settings.yml`

config/locales/simple_form.en.yml

@@ -48,6 +48,8 @@ en:
         text: This will help us review your application
       sessions:
         otp: 'Enter the two-factor code generated by your phone app or use one of your recovery codes:'
+      tag:
+        name: You can only change the casing of the letters, for example, to make it more readable
       user:
         chosen_languages: When checked, only toots in selected languages will be displayed in public timelines
     labels:
@@ -137,6 +139,11 @@ en:
         pending_account: Send e-mail when a new account needs review
         reblog: Send e-mail when someone boosts your status
         report: Send e-mail when a new report is submitted
+        trending_tag: Send e-mail when an unreviewed hashtag is trending
+      tag:
+        listable: Allow this hashtag to appear on the profile directory
+        trendable: Allow this hashtag to appear under trends
+        usable: Allow toots to use this hashtag
     'no': 'No'
     recommended: Recommended
     required: