Add validations to admin settings (#10348)

* Add validations to admin settings

- Validate correct HTML markup
- Validate presence of contact username & e-mail
- Validate that all usernames are valid
- Validate that enums have expected values

* Fix code style issue

* Fix tests

config/locales/en.yml

@@ -586,6 +586,9 @@ en:
       content: We're sorry, but something went wrong on our end.
       title: This page is not correct
     noscript_html: To use the Mastodon web application, please enable JavaScript. Alternatively, try one of the <a href="%{apps_path}">native apps</a> for Mastodon for your platform.
+  existing_username_validator:
+    not_found: could not find a local user with that username
+    not_found_multiple: could not find %{usernames}
   exports:
     archive_takeout:
       date: Date
@@ -633,6 +636,8 @@ en:
     validation_errors:
       one: Something isn't quite right yet! Please review the error below
       other: Something isn't quite right yet! Please review %{count} errors below
+  html_validator:
+    invalid_markup: contains invalid HTML markup
   identity_proofs:
     active: Active
     authorize: Yes, authorize

config/navigation.rb

@@ -37,7 +37,7 @@ SimpleNavigation::Configuration.run do |navigation|
 
     primary.item :admin, safe_join([fa_icon('cogs fw'), t('admin.title')]), admin_dashboard_url, if: proc { current_user.staff? } do |admin|
       admin.item :dashboard, safe_join([fa_icon('tachometer fw'), t('admin.dashboard.title')]), admin_dashboard_url
-      admin.item :settings, safe_join([fa_icon('cogs fw'), t('admin.settings.title')]), edit_admin_settings_url, if: -> { current_user.admin? }
+      admin.item :settings, safe_join([fa_icon('cogs fw'), t('admin.settings.title')]), edit_admin_settings_url, if: -> { current_user.admin? }, highlights_on: %r{/admin/settings}
       admin.item :custom_emojis, safe_join([fa_icon('smile-o fw'), t('admin.custom_emojis.title')]), admin_custom_emojis_url, highlights_on: %r{/admin/custom_emojis}
       admin.item :relays, safe_join([fa_icon('exchange fw'), t('admin.relays.title')]), admin_relays_url, if: -> { current_user.admin? }, highlights_on: %r{/admin/relays}
       admin.item :subscriptions, safe_join([fa_icon('paper-plane-o fw'), t('admin.subscriptions.title')]), admin_subscriptions_url, if: -> { current_user.admin? }