Savable custom filters

I think it would be a nice feature to have something like custom categories that users can create themselves.

For example a filter for some "light reading on the road":
* Reading time < 5 minutes.
* Tags: news.

Some of the criteria that should be included in the filters:
* Reading time.
* Tags.
* Domain?
* Search string?
* Favorite?

No files, tests, or entry points are named. Start by locating the existing filter and category flow, then determine how reading time, tags, domain, search, and favorites can be combined and persisted. Done means users can define, save, and reuse a custom filter with an agreed set of criteria.

Written by the indexing model from the issue text.