Add MyMLH Integration

Feature Request: [MyMLH](https://my.mlh.io/) is an OAuth provider for student hackathon registration. It would allow hackers who have been to previous MLH Member events or used the Hardware Lab to easily sign up for other events without re-entering all of their information. I'd love to see it added as a login provider here.

No files or tests are named. Start by locating the dashboard's existing login-provider entry point and reviewing how OAuth providers are integrated; use MyMLH's OAuth documentation to identify the required flow. Done means users can authenticate with MyMLH and complete the existing registration path.

Written by the indexing model from the issue text.