Provide a way to manually do database cleanups

[codeling]

There should be a clearer way to clear up old failed login database entries:

• In the list, there could be a button triggering a dialog asking for a maximum age of entries to keep
• Also, next to the list, there could be a hint pointing at the setting for automatically deleting old entries

Inspired by comments from https://github.com/codeling/bfstop/issues/196.