adds cleanup functionality for old devices

- device now last field 'last connection' which gets updated
  at every server connection from the respective device
- manual cleanup script in database deletes all devices which
  have never been updated, or have a timestamp older than 30 days

database/migrations/versions/121eab3835ee_.py

@@ -0,0 +1,28 @@
+"""empty message
+
+Revision ID: 121eab3835ee
+Revises: e5ef5e4a807b
+Create Date: 2020-07-17 08:03:36.947158
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = '121eab3835ee'
+down_revision = 'e5ef5e4a807b'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('device', sa.Column('lastConnection', sa.BigInteger(), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('device', 'lastConnection')
+    # ### end Alembic commands ###