Current name - page_heading_actions is long and hard to remember,
this commit shortens it to just heading_actions, which is way more
easier to remember and write, it's logical too.

When you have many filters, it may be hard for you to reach the button
to create yet another one. This commit moves creation button to the
heading, leaving the page just for the list.

On the other hand, when there are no filters, page looks kind of
strange with the empty table. So text stating obvious fact that user
has no filters was added in this commit too.

Closes #11020
Closes #12790

* Add keyword filtering

    GET|POST       /api/v1/filters
    GET|PUT|DELETE /api/v1/filters/:id

- Irreversible filters can drop toots from home or notifications
- Other filters can hide toots through the client app
- Filters use a phrase valid in particular contexts, expiration

* Make sure expired filters don't get applied client-side

* Add missing API methods

* Remove "regex filter" from column settings

* Add tests

* Add test for FeedManager

* Add CustomFilter test

* Add UI for managing filters

* Add streaming API event to allow syncing filters

* Fix tests