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.

test/functional/bans_controller_test.rb

@@ -60,6 +60,7 @@ class BansControllerTest < ActionDispatch::IntegrationTest
 
           assert_redirected_to bans_path
           assert_equal(true, @user.reload.is_banned?)
+          assert_match(/banned <@#{@user.name}> 1 day: xxx/, ModAction.last.description)
         end
       end
 
@@ -131,7 +132,9 @@ class BansControllerTest < ActionDispatch::IntegrationTest
 
         assert_difference("Ban.count", -1) do
           delete_auth ban_path(@ban.id), @mod
+
           assert_redirected_to bans_path
+          assert_match(/unbanned <@#{@ban.user.name}>/, ModAction.last.description)
         end
       end
     end