authz: utilize expired_at and last_valid_at columns of user_external_accounts

Created by: unknwon

This PR does two things in the PermsSyncer:

• When successfully used a token from user_external_accounts, we update the last_valid_at column.
• Otherwise, update the expired_at column.

Part of #14899

TODOs

• Implements errcode.IsUnauthorized for GitHub and GitLab errors
• Add tests for db.ExternalAccounts
• Add tests for PermsSyncer