Separate out some functions

`co-authors-plus.php` is pretty big and unnecessarily so. It should be split up into more manageable chunks.

For example, `filter_ef_calendar_item_information_fields()` and `filter_ef_story_budget_term_column_value()` could move into the `integrations` folder, along with the AMP compat stuff.

Admin UI functionality could be split into a separate file/class too.

Start with co-authors-plus.php and inventory its functions, then inspect the integrations folder. Review filter_ef_calendar_item_information_fields(), filter_ef_story_budget_term_column_value(), the AMP compatibility code, and the admin UI functionality to identify sensible boundaries. Done means these responsibilities are separated into manageable files or classes without changing existing behavior.

Written by the indexing model from the issue text.