Clean up inactive counters

Backend should run a regular job to clean up inactive counters.

This should probably work in stages: Stage 1: mark counters that have not been modified as 'inactive' Stage 2: remove inactive counters

A counter should be marked inactive:

If the counter has no increments at all, it should be marked inactive after some weeks
If the counter has been created by an unknown user and not been modified for a few months
If the counter has been created by a logged ing user and not been modified for a few months

Inactive counters shall be permanently removed a few weeks after becoming inactive. Inactive flag will be reset if the counter is modified.

Users shall have the possibility to additionally mark a counter as 'archived' thus preventing its deletion after becoming inactive.

skuzzle / cmp