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

config/routes.rb

@@ -423,9 +423,10 @@ Rails.application.routes.draw do
 
   get '/web/(*any)', to: 'home#index', as: :web
 
-  get '/about',      to: 'about#show'
-  get '/about/more', to: 'about#more'
-  get '/terms',      to: 'about#terms'
+  get '/about',        to: 'about#show'
+  get '/about/more',   to: 'about#more'
+  get '/about/blocks', to: 'about#blocks'
+  get '/terms',        to: 'about#terms'
 
   root 'home#index'