Automatically delete stale image uploads

[bates64]

Furthering #130.

There needs to be a config option for how long image uploads should exist on the server for, probably defaulting to, like, 3 months. That means a mini date parser is in order - to change 2 months to 30 * 24 * 60 * 60 * 1000, for example.

> set delete_images_after 2 months
Set.

Proposed implementation (@towerofnix, #115):

Assuming the server stays persistently running, just remove every message upload with a "to be deleted on (date)" less than the current date every 30 minutes or such.

(This relates to actually deleting deleted messages from the server - see #115)

[bates64]

Suggestion: option to upload to pomf.cat rather than the server.