Add GitHub action to auto-rebase the migration branch

[cgsunkel]

Description of change

In order to keep the migration-deploy branch up to date we are required to rebase it against main every day. The current expectation is either for a developer to manually rebase the branch every morning, or for all developers to rebase the branch after merging a PR.

This isn't ideal so I have attempted to create a GitHub action to perform this process automatically. It will run each time a PR is merged into this repo's main branch. Once we have fully migrated the API on to the DBT Platform, this action should be removed.

You can test the action by doing the following:

• Open these two test branches in separate tabs - chore/github-action-main and chore/github-action-mig
• Open a PR to update the README in chore/github-action-main and merge it
• Open the Actions tab. You should see a running action called Update Migration Branch
• Once this is completed, switch to chore/github-action-mig and refresh. You should see the changes from your PR.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.46%. Comparing base (764ea4f) to head (911a6f8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5315      +/-   ##
==========================================
- Coverage   96.47%   96.46%   -0.01%     
==========================================
  Files         925      925              
  Lines       21906    21906              
  Branches     1961     1961              
==========================================
- Hits        21133    21131       -2     
- Misses        625      626       +1     
- Partials      148      149       +1     

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