Custom field filtered search tokens for issues

What does this MR do and why?

Adds custom field to filtered search dropdown on issue list.

For [Custom Fields] Filter by select custom fields (#479573 - closed) & #515950 (closed)

How to test

1. Enable custom_fields_feature feature flag
2. Go to Settings > Issues for top level group
3. Add a custom field. Type should be Select or Multi select, Work Item should include Issue
4. Go to issue list page for the group (and/or a sub-group or project)
5. Click in filtered search bar to see your custom field listed in the dropdown

Note that clicking the field does not do anything yet