Create an initial Alembic migration

This way the database will get stamped, and Git will create the
`versions` directory without which Alembic won’t work.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

migrations/versions/f47cad5d6d03_initial_revision.py

+"""initial revision
+
+Revision ID: f47cad5d6d03
+Revises: 
+Create Date: 2020-02-23 13:23:32.331396
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'f47cad5d6d03'
+down_revision = None
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    pass
+
+
+def downgrade():
+    pass