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/unit/post_vote_test.rb

@@ -106,7 +106,7 @@ class PostVoteTest < ActiveSupport::TestCase
         vote = create(:post_vote, post: @post, score: 1)
 
         vote.soft_delete!(updater: admin)
-        assert_match(/admin deleted post vote #\d+ on post #\d+/, ModAction.post_vote_delete.last.description)
+        assert_match(/deleted post vote #\d+ on post #\d+/, ModAction.post_vote_delete.last.description)
       end
     end
 
@@ -128,7 +128,7 @@ class PostVoteTest < ActiveSupport::TestCase
       end
 
       should "leave a mod action" do
-        assert_match(/admin undeleted post vote #\d+ on post #\d+/, ModAction.post_vote_undelete.last.description)
+        assert_match(/undeleted post vote #\d+ on post #\d+/, ModAction.post_vote_undelete.last.description)
       end
     end
   end