Add an action to bulk delete imported items

I've worked some myself on this feature. Was a bit more complicated than I thought. I refactored some code too in the process. Test coverage is added for clearing feeds (delete imported items), but since I touched code for the existing delete feed action, that will need test coverage too.

Next steps:

• Finish and clean up the clear feed action (I assume some tests will fail as of now).
• Add test coverage for delete feed action.
• Add an update function for installing new config "system.action.feeds_feed_clear_action.yml".

This is ready for review.

Merged the code!

Automatically closed - issue fixed for 2 weeks with no activity.