mod actions: fix messages to use consistent format.

Fix mod actions to use the same message format everywhere.

Before mod actions were formatted in various inconsistent ways:

* "deleted post #1234"
* "comment #1234 updated by <user>"
* "<user> updated forum #1234"
* "<user> level changed Member -> Builder"

Now all mod actions consistently use this format:

* "deleted post #1234"
* "updated comment #1234"
* "updated forum #1234"
* "promoted <user> from Member to Builder"

This way mod actions are formatted consistently with other actions on
the /user_actions page, where everything is written as "<user> did X".

Also add a fix script to fix existing mod actions.

app/models/ban.rb

@@ -82,11 +82,11 @@ class Ban < ApplicationRecord
   end
 
   def create_ban_mod_action
-    ModAction.log(%{Banned <@#{user_name}> #{humanized_duration}: #{reason}}, :user_ban)
+    ModAction.log(%{banned <@#{user_name}> #{humanized_duration}: #{reason}}, :user_ban, banner)
   end
 
   def create_unban_mod_action
-    ModAction.log(%{Unbanned <@#{user_name}>}, :user_unban)
+    ModAction.log(%{unbanned <@#{user_name}>}, :user_unban)
   end
 
   def self.available_includes