dvr: Add option to automatically delete recording after playback.

dvr: Add option to automatically delete recording after playback.

Previously when watching a programme, the user usually has to then
manually delete the programme to recover disk space, or wait for its
retention to expire.

So we now add an option to Config->Recording->DVR Profile. This allows
the user to select time after watching to automatically delete the
recording (unless it is marked as "keep forever"). Default is disabled
(do not delete after playback).

For example, if the user specifies "2 days" then we'd delete the
recording two days after playback, even if the retention period is "3
months".

"Playback" can vary based on client. Some clients read and cache the
entire file before starting playback, so the file would be marked as
watched immediately. Other clients only buffer a small amount, so the
file will be marked as watched near the end of the show.