Merge branch 'main' into glitch-soc/merge-upstream

Conflicts:
- `app/controllers/home_controller.rb`:
  Upstream made it so `/web` is available to non-logged-in users
  and `/` redirects to `/web` instead of `/about`.
  Kept our version since glitch-soc's WebUI doesn't have what's
  needed yet and I think /about is still a much better landing
  page anyway.
- `app/models/form/admin_settings.rb`:
  Upstream added new settings, and glitch-soc had an extra setting.
  Not really a conflict.
  Added upstream's new settings.
- `app/serializers/initial_state_serializer.rb`:
  Upstream added a new `server` initial state object.
  Not really a conflict.
  Merged upstream's changes.
- `app/views/admin/settings/edit.html.haml`:
  Upstream added new settings.
  Not really a conflict.
  Merged upstream's changes.
- `app/workers/scheduler/feed_cleanup_scheduler.rb`:
  Upstream refactored that part and removed the file.
  Ported our relevant changes into `app/lib/vacuum/feeds_vacuum.rb`
- `config/settings.yml`:
  Upstream added new settings.
  Not a real conflict.
  Added upstream's new settings.

app/validators/language_validator.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class LanguageValidator < ActiveModel::EachValidator
+  include LanguagesHelper
+
+  def validate_each(record, attribute, value)
+    record.errors.add(attribute, :invalid) unless valid?(value)
+  end
+
+  private
+
+  def valid?(str)
+    if str.nil?
+      true
+    elsif str.is_a?(Array)
+      str.all? { |x| valid_locale?(x) }
+    else
+      valid_locale?(str)
+    end
+  end
+end