Add public blocks to /about/blocks (#11298)

* Add automatic blocklist display in /about/blocks

Inspired by https://github.com/Gargron/mastodon.social-misc

* Add admin option to set who can see instance blocks

* Normalize locales files

* Rename “Sandbox” to “Silence” for consistency

* Disable /about/blocks when in whitelist mode

* Optionally display rationale for domain blocks

* Only display domain blocks that have user-facing limitations, and order them

* Redesign table of blocked domains to better handle long domain names and rationales

* Change domain blocks ordering now that rationales aren't displayed right away

* Only show explanation for block severities actually in use

* Reword instance block explanations and add disclaimer for public fetch mode

app/models/form/admin_settings.rb

@@ -30,6 +30,8 @@ class Form::AdminSettings
     mascot
     spam_check_enabled
     trends
+    show_domain_blocks
+    show_domain_blocks_rationale
   ).freeze
 
   BOOLEAN_KEYS = %i(
@@ -60,6 +62,8 @@ class Form::AdminSettings
   validates :site_contact_email, :site_contact_username, presence: true
   validates :site_contact_username, existing_username: true
   validates :bootstrap_timeline_accounts, existing_username: { multiple: true }
+  validates :show_domain_blocks, inclusion: { in: %w(disabled users all) }
+  validates :show_domain_blocks_rationale, inclusion: { in: %w(disabled users all) }
 
   def initialize(_attributes = {})
     super