Add button clean up database button/route

[fgypas]

Add button/route under "Show all workflows" that cleans up the database. It would be nice if we can to ask for confirmation, as people might press it by accident.

[fgypas]

~~Blocked by #90~~

[dafentoulis]

I'm not sure I understand where the button should appear, something like this maybe (top-right)?

[vsoch]

Maybe put these kinds of buttons where they are less likely to be clicked - under the table bottom left, or top left alongside the pagination? There are also custom buttons you can add to the table: https://datatables.net/extensions/buttons/examples/initialisation/custom.html

The image shown above makes the cleanup bar (with "All Workflows") look like a very large button, so if that spot is the choice maybe it would make sense to show some boundary between the two. :)