Add trending links (#16917)

* Add trending links

* Add overriding specific links trendability

* Add link type to preview cards and only trend articles

Change trends review notifications from being sent every 5 minutes to being sent every 2 hours

Change threshold from 5 unique accounts to 15 unique accounts

* Fix tests

config/locales/simple_form.en.yml

@@ -204,8 +204,8 @@ en:
         mention: Someone mentioned you
         pending_account: New account needs review
         reblog: Someone boosted your post
-        report: New report is submitted
-        trending_tag: An unreviewed hashtag is trending
+        report: A new report is submitted
+        trending_tag: A new trend requires approval
       rule:
         text: Rule
       tag: