We're planning to implement a feature that allows adding required fields for existing users. This PR does some preparatory refactoring to make that possible. There should be no changes to existing behaviour. Just a small update to the admin UI.
- Add plugin outlet to `AdminUserFieldItem`
- Add ability to include custom fields when saving `AdminUserFieldItem`
- Update plugin API with `includeUserFieldPropertiesOnSave` per ☝️
- Add `DiscoursePluginRegistry` to `UserFieldsController` to add custom columns
When the admin creates a new custom field they can specify if that field should be searchable or not.
That setting is taken into consideration for quick search results.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
