Use GitHub user ids instead of usernames for authentication

Otherwise, renaming a GitHub account could allow someone else
to gain access to a CDB account.

migrations/versions/1fe2e44cf565_.py

@@ -0,0 +1,28 @@
+"""empty message
+
+Revision ID: 1fe2e44cf565
+Revises: d73078c5d619
+Create Date: 2024-03-30 16:19:47.384716
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision = '1fe2e44cf565'
+down_revision = 'd73078c5d619'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    with op.batch_alter_table('user', schema=None) as batch_op:
+        batch_op.add_column(sa.Column('github_user_id', sa.Integer(), nullable=True))
+        batch_op.create_unique_constraint("_user_github_user_id", ['github_user_id'])
+
+
+def downgrade():
+    with op.batch_alter_table('user', schema=None) as batch_op:
+        batch_op.drop_constraint("_user_github_user_id", type_='unique')
+        batch_op.drop_column('github_user_id')