Strategy for pruning old builds

[agis]

There should be a strategy for somehow pruning old builds. This could be in the form of a "retention period" configuration.

This is necessary, otherwise disk usage will grow indefinitely.

[agis]

We now have a script for doing this. Ideally, we want the server to do this automatically. One should be able to configure how much usage each project should consume and how much total usage all projects should consume.