retention policy

We think anout introducing a period, after which pads and images will be deleted using the plugin

  ep_delete_after_delay

The default time could be set to e.g. 3 years of inactivity at a pad.

Well, some people use board.net e.g. to plan anniversary celebrations, that may happen every 5 years - they would loose the feedback and wishes see e.g. https://board.net/p/r.7915c3db5e7896d2bd9d8fbbca56d159

Ideally a user could tag a board that it will be never deleted see https://framagit.org/fiat-tux/etherpad/ep_delete_after_delay/-/issues/12

We also need to add to the plugin that images are being deleted.