FIX: use a custom prefix for custom flags (#28839)

Currently, when the custom flag has the same name as the system flag (which is disabled) then it is not displayed. To fix the problem, `custom_` prefix as `name_key` is used to distinguish between the system and the custom flag.

I considered writing a migration to fix existing custom flags name key. However, at the end of migration I would need to run rails code to reset cache `Flag.reset_flag_settings!`. I decided to skip that step as it is a very edge case. If someone has the same flag name as the system flag, then all they have to do is edit the flag and click save.

In addition, I made 2 small fixes:
- edit flag title was missing translation;
- flag form UI was not showing that description is the required field.

spec/models/post_action_spec.rb

@@ -791,7 +791,7 @@ RSpec.describe PostAction do
         PostActionCreator.new(
           Discourse.system_user,
           post,
-          PostActionType.types[:flag_without_message],
+          PostActionType.types[:custom_flag_without_message],
           message: "WAT",
         ).perform
 
@@ -824,7 +824,7 @@ RSpec.describe PostAction do
         PostActionCreator.new(
           Discourse.system_user,
           post,
-          PostActionType.types[:flag_with_message],
+          PostActionType.types[:custom_flag_with_message],
           message: "WAT",
         ).perform