Add an action to bulk delete imported items

Problem/Motivation

Removing items related with feed entries en masse to easily prepare multiple feeds for a reimport.

Proposed resolution

An action plugin and a kernel test that covers the action plugin should be written.

The action plugin

Add a file to src/Plugin/Action and implement an action plugin that triggers the deletion of all imported items for every feed that is passed to the action plugin.

The kernel test

The kernel test could be written as follows:

1. Create feed type. I think using the parser "syndication" (for parsing RSS feeds) is the easiest for this test.
2. Create two feeds: one that imports drupalplanet.rss2 and another one that imports googlenewstz.rss2. These RSS feeds are found in tests/resources/rss.
3. Import content for both feeds.
4. Assert that content has been imported.
5. Apply the action plugin.
6. Assert that all content was deleted.

Remaining tasks

1. Write an action plugin
2. Write a kernel test that covers the action plugin
3. Finish and clean up the clear feed action (I assume some tests will fail as of now).
4. Add test coverage for delete feed action.
5. Add an update function for installing new config "system.action.feeds_feed_clear_action.yml".

User interface changes

New entry in the 'With selection' dropdown menu: