Remove the terms blacklist and whitelist from UX (#14149)

Localization strings:

- "Whitelist mode" -> "Limited federation mode"
- "Blacklist e-mail domain" -> "Block e-mail domain"
- "Whitelist domain" -> "Allow domain for federation"

...And so on

Environment variables (backwards-compatible):

- `WHITELIST_MODE` -> `LIMITED_FEDERATION_MODE`
- `EMAIL_DOMAIN_BLACKLIST` -> `EMAIL_DOMAIN_DENYLIST`
- `EMAIL_DOMAIN_WHITELIST` -> `EMAIL_DOMAIN_ALLOWLIST`

tootctl:

- `tootctl domains purge --whitelist-mode` -> `tootctl domains purge --limited-federation-mode`

Removed badly maintained and no longer relevant .env.production.sample file

config/initializers/2_whitelist_mode.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 Rails.application.configure do
-  config.x.whitelist_mode = ENV['WHITELIST_MODE'] == 'true'
+  config.x.whitelist_mode = (ENV['LIMITED_FEDERATION_MODE'] || ENV['WHITELIST_MODE']) == 'true'
 end