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/layouts/application.html.haml

@@ -2,7 +2,7 @@
 %html{ lang: I18n.locale }
   %head
     %meta{ charset: 'utf-8' }/
-    %meta{ name: 'viewport', content: 'width=device-width, initial-scale=1' }/   
+    %meta{ name: 'viewport', content: 'width=device-width, initial-scale=1' }/
     %link{ rel: 'icon', href: favicon_path, type: 'image/x-icon' }/
     %link{ rel: 'apple-touch-icon', sizes: '180x180', href: '/apple-touch-icon.png' }/
     %link{ rel: 'mask-icon', href: '/mask-icon.svg', color: '#2B90D9' }/
@@ -11,11 +11,7 @@
     %meta{ name: 'theme-color', content: '#282c37' }/
     %meta{ name: 'apple-mobile-web-app-capable', content: 'yes' }/
 
-    %title<
-      - if content_for?(:page_title)
-        = yield(:page_title)
-        = ' - '
-      = title
+    %title= content_for?(:page_title) ? safe_join([yield(:page_title).chomp, ' - ', title]) : title
 
     = stylesheet_pack_tag 'common', media: 'all'
     = stylesheet_pack_tag current_theme, media: 'all'