Ignored columns can only be dropped when its associated post-deploy
migration has been promoted to a regular migration. This is so because
Discourse doesn't rely on a schema file system to setup a brand new
database and thus the column information will be loaded by the
application first before the post-deploy migration runs.
Skipping methods we don't use gives us mem/perf gains (minuscule but still), but more importantly fixes warnings about `Poll#open` (created by `enum :status`) conflicting with some internal AR method. 😃
The first thing we needed here was an enum rather than a boolean to determine how a directory_column was created. Now we have `automatic`, `user_field` and `plugin` directory columns.
This plugin API is assuming that the plugin has added a migration to a column to the `directory_items` table.
This was created to be initially used by discourse-solved. PR with API usage - https://github.com/discourse/discourse-solved/pull/137/
