Merge commit '34cd7d6585992c03298c175ab5d22ad059b58cdb' into glitch-soc/merge-upstream

Conflicts:
- `CONTRIBUTING.md`:
  Upstream changed the file, while we had a different one.
  Updated the common parts.
- `README.md`:
  Upstream changed the file, while we had a different one.
  Updated the common parts.
- `app/helpers/application_helper.rb`:
  Upstream added helpers where glitch-soc had extra ones.
  Added upstream's new helpers.
- `app/models/form/admin_settings.rb`:
  Upstream added some custom handling of one setting, while
  glitch-soc had additional code.
  Ported upstream's code.
- `lib/mastodon/version.rb`:
  Upstream moved some things to `config/mastodon.yml`.
  Did the same.
- `spec/requests/api/v1/accounts/credentials_spec.rb`:
  I don't know honestly.

app/helpers/application_helper.rb

@@ -239,6 +239,14 @@ module ApplicationHelper
     I18n.t 'user_mailer.welcome.hashtags_recent_count', people: number_with_delimiter(people), count: people
   end
 
+  def app_store_url_ios
+    'https://apps.apple.com/app/mastodon-for-iphone-and-ipad/id1571998974'
+  end
+
+  def app_store_url_android
+    'https://play.google.com/store/apps/details?id=org.joinmastodon.android'
+  end
+
   # glitch-soc addition to handle the multiple flavors
   def preload_locale_pack
     supported_locales = Themes.instance.flavour(current_flavour)['locales']