Add featured hashtags to profiles (#9755)

* Add hashtag filter to profiles

GET /@:username/tagged/:hashtag
GET /api/v1/accounts/:id/statuses?tagged=:hashtag

* Display featured hashtags on public profile

* Use separate model for featured tags

* Update featured hashtag counters on-write

* Limit featured tags to 10

config/locales/simple_form.en.yml

@@ -37,6 +37,8 @@ en:
         setting_theme: Affects how Mastodon looks when you're logged in from any device.
         username: Your username will be unique on %{domain}
         whole_word: When the keyword or phrase is alphanumeric only, it will only be applied if it matches the whole word
+      featured_tag:
+        name: 'You might want to use one of these:'
       imports:
         data: CSV file exported from another Mastodon instance
       sessions:
@@ -110,6 +112,8 @@ en:
         username: Username
         username_or_email: Username or Email
         whole_word: Whole word
+      featured_tag:
+        name: Hashtag
       interactions:
         must_be_follower: Block notifications from non-followers
         must_be_following: Block notifications from people you don't follow