Refactor 'metrics' database table and datasource_metrics worker

[sal-uva]

Since 4CAT will no longer be used to locally store data, the information that is now stored in the 'metrics' table (which was used to e.g. keep track of counts of posts per day) should be moved to 4cat-scrapers.

[stijn-uva]

The metrics table is also used to cache metrics relating to 4CAT's use of disk space. It's not a perfect fit for that, so the table could be refactored if it doesn't need to store actual datasource-specific metrics anymore, but it shouldn't be deleted because this functionality needs to remain inside 4CAT itself.