FIX: Don't store translated trust level names in anonymous cache (#13224)

Refactors `TrustLevel` and moves translations from server to client

Additional changes:
  * "staff" and "admin" wasn't translatable in site settings
  * it replaces a concatenated string with a translation
  * uses translation for trust levels in users_by_trust_level report
  * adds a DB migration to rename keys of translation overrides affected by this commit

lib/trust_level.rb

@@ -4,8 +4,6 @@ class InvalidTrustLevel < StandardError; end
 
 class TrustLevel
 
-  attr_reader :id, :name
-
   class << self
 
     def [](level)
@@ -17,12 +15,6 @@ class TrustLevel
       @levels ||= Enum.new(:newuser, :basic, :member, :regular, :leader, start: 0)
     end
 
-    def all
-      levels.map do |name_key, id|
-        TrustLevel.new(name_key, id)
-      end
-    end
-
     def valid?(level)
       valid_range === level
     end
@@ -35,15 +27,9 @@ class TrustLevel
       (current_level || 0) >= level
     end
 
-  end
-
-  def initialize(name_key, id)
-    @name = I18n.t("trust_levels.#{name_key}.title")
-    @id = id
-  end
-
-  def serializable_hash
-    { id: @id, name: @name }
+    def name(level)
+      I18n.t("js.trust_levels.names.#{levels[level]}")
+    end
   end
 
 end