Add support for expiring files

SUPPLEMENTALLY:
- Add an `expiration` field to the `file` table of the database
- Produce a migration for the above change
- Overhaul the cleanup script, and integrate into fhost.py
  (now run using FLASK_APP=fhost flask prune)
- Replace the old cleanup script with a deprecation notice
- Add information about how to expire files to the index
- Update the README with information about the new script

Squashed commits:

Add a note explaining that expired files aren't immediately removed

Show correct times on the index page graph

Improve the migration script, removing the need for --legacy

Use automap in place of an explicit file map in migration

Remove vestigial `touch()`

Don't crash when upgrading a fresh database

Remove vestigial warning about legacy files

More efficiently filter to unexpired files when migrating

https://git.0x0.st/mia/0x0/pulls/72#issuecomment-224

Coalesce updates to the database during migration

https://git.0x0.st/mia/0x0/pulls/72#issuecomment-226

Remove vestigial database model

https://git.0x0.st/mia/0x0/pulls/72#issuecomment-261

prune:  Stream expired files from the database

(as opposed to collecting them all first)

config.example.py:  Add min & max expiration + description

README.rst

@@ -35,8 +35,8 @@ downsides, one of them being that range requests will not work. This is a
 problem for example when streaming media files: It won’t be possible to seek,
 and some ISOBMFF (MP4) files will not play at all.
 
-To make files expire, simply create a cronjob that runs ``cleanup.py`` every
-now and then.
+To make files expire, simply create a cronjob that runs ``FLASK_APP=fhost
+flask prune`` every now and then.
 
 Before running the service for the first time, run ``FLASK_APP=fhost flask db upgrade``.