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

cleanup.py

@@ -1,44 +1,8 @@
 #!/usr/bin/env python3
 
-"""
-    Copyright © 2020 Mia Herkt
-    Licensed under the EUPL, Version 1.2 or - as soon as approved
-    by the European Commission - subsequent versions of the EUPL
-    (the "License");
-    You may not use this work except in compliance with the License.
-    You may obtain a copy of the license at:
-
-        https://joinup.ec.europa.eu/software/page/eupl
-
-    Unless required by applicable law or agreed to in writing,
-    software distributed under the License is distributed on an
-    "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
-    either express or implied.
-    See the License for the specific language governing permissions
-    and limitations under the License.
-"""
-
-import os
-import sys
-import time
-import datetime
-from fhost import app
-
-os.chdir(os.path.dirname(sys.argv[0]))
-os.chdir(app.config["FHOST_STORAGE_PATH"])
-
-files = [f for f in os.listdir(".")]
-
-maxs = app.config["MAX_CONTENT_LENGTH"]
-mind = 30
-maxd = 365
-
-for f in files:
-    stat = os.stat(f)
-    systime = time.time()
-    age = datetime.timedelta(seconds=(systime - stat.st_mtime)).days
-
-    maxage = mind + (-maxd + mind) * (stat.st_size / maxs - 1) ** 3
-
-    if age >= maxage:
-        os.remove(f)
+print("This script has been replaced!!")
+print("Instead, please run")
+print("")
+print("   $ FLASK_APP=fhost flask prune")
+print("")
+exit(1);