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/post.rb

@@ -1462,7 +1462,7 @@ class Post < ApplicationRecord
       if category == "iqdb"
         update_iqdb
 
-        ModAction.log("<@#{user.name}> regenerated IQDB for post ##{id}", :post_regenerate_iqdb, user)
+        ModAction.log("regenerated IQDB for post ##{id}", :post_regenerate_iqdb, user)
       else
         media_file = media_asset.variant(:original).open_file
         media_asset.distribute_files!(media_file)
@@ -1484,7 +1484,7 @@ class Post < ApplicationRecord
         purge_cached_urls!
         update_iqdb
 
-        ModAction.log("<@#{user.name}> regenerated image samples for post ##{id}", :post_regenerate, user)
+        ModAction.log("regenerated image samples for post ##{id}", :post_regenerate, user)
       end
     end