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

1 files changed, 13 insertions, 0 deletions

diff --git a/instance/config.example.py b/instance/config.example.py
index 64c977c..019ec11 100644
--- a/instance/config.example.py
+++ b/instance/config.example.py
@@ -45,6 +45,19 @@ MAX_CONTENT_LENGTH = 256 * 1024 * 1024 # Default: 256MiB
 MAX_URL_LENGTH = 4096
 
 
+# The minimum and maximum amount of time we'll retain a file for
+#
+# Small files (nearing zero bytes) are stored for the longest possible expiration date,
+# while larger files (nearing MAX_CONTENT_LENGTH bytes) are stored for the shortest amount
+# of time.  Values between these two extremes are interpolated with an exponential curve,
+# like the one shown on the index page.
+#
+# All times are in milliseconds.  If you want all files to be stored for the same amount
+# of time, set these to the same value.
+FHOST_MIN_EXPIRATION = 30  * 24 * 60 * 60 * 1000
+FHOST_MAX_EXPIRATION = 365 * 24 * 60 * 60 * 1000
+
+
 # Use the X-SENDFILE header to speed up serving files w/ compatible webservers
 #
 # Some webservers can be configured use the X-Sendfile header to handle sending