Add missing meta description to profiles, some other SEO stuff (#6706)

- Add missing meta description to profiles
- Add canonical rel link to landing page
- Remove linebreaks from title tags
- Add username to profile title
- Add toots/following/followers to profile description tags
- Add next/prev rel links to profiles
- Do not index follower/following variants of profiles

app/views/following_accounts/index.html.haml

@@ -2,9 +2,7 @@
   = t('accounts.people_followed_by', name: display_name(@account))
 
 - content_for :header_tags do
-  - if @account.user&.setting_noindex
-    %meta{ name: 'robots', content: 'noindex' }/
-
+  %meta{ name: 'robots', content: 'noindex' }/
   = render 'accounts/og', account: @account, url: account_followers_url(@account, only_path: false)
 
 = render 'accounts/header', account: @account