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

migrations/versions/939a08e1d6e5_.py

@@ -0,0 +1,22 @@
+"""add file expirations [creates legacy files]
+
+Revision ID: 939a08e1d6e5
+Revises: 7e246705da6a
+Create Date: 2022-11-22 12:16:32.517184
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '939a08e1d6e5'
+down_revision = '7e246705da6a'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.add_column('file', sa.Column('expiration', sa.BigInteger()))
+
+
+def downgrade():
+    op.drop_column('file', 'expiration')