Fix regressions from #2683 (#2970)

* Fix regressions from #2683

Properly format spoiler text HTML, while keeping old logic for blankness intact
Process hashtags and mentions in spoiler text
Format spoiler text for Atom
Change "show more" toggle into a button instead of anchor
Fix style regression on dropdowns for detailed statuses

* Fix lint issue

* Convert spoiler text to plaintext in desktop notifications

app/services/process_mentions_service.rb

@@ -10,7 +10,9 @@ class ProcessMentionsService < BaseService
   def call(status)
     return unless status.local?
 
-    status.text.scan(Account::MENTION_RE).each do |match|
+    text = [status.text, status.spoiler_text].reject(&:blank?).join(' ')
+
+    text.scan(Account::MENTION_RE).each do |match|
       username, domain  = match.first.split('@')
       mentioned_account = Account.find_remote(username, domain)