Add pruning function based on tag creation date in Quay

Currently, no pruning functions exist in Quay. This is a problem for clients who cannot use the label quay.expires-after because they don't have a regular release cycle. The only way to prune images today is to use a script and several API commands that would first list tags in a repository and then delete each tag which is older than a specific date. It would be a good idea to add such a pruning function to repository settings so that images older than a certain date be automatically deleted from the repo. Editing of these settings should be allowed only for repo admins and should be audited.