feat: add GitLab OAuth client

[almereyda]

This adds a GitLab OAuth client, based on the Google and GitHub examples.

To raise exceptions when OAuth is malconfigured, it also adds an additional OAUTH_NOT_CONFIGURED error.

There is a bit of code duplication between admin, web and spaces, but that may be a design choice.

This has been tested successfully against a custom GitLab instance and should work similarly against the default instance.

addresses #408
addresses #413

[CLAassistant]

All committers have signed the CLA.

[almereyda]

As it turns out, I didn't find a place in the repository to document the added environment variables.

Seems good to accompany this PR with a parallel one in makeplane/docs.

[almereyda]

Rebased to accommodate 36b82a7

[sriramveeraghanta]

@almereyda

Thank you for your contribution. We have merged your pull request into an internal branch for validation and testing. We will soon merge these changes into the master branch along with the updates for our next release.

[almereyda]

Thanks! I'm very excited.

Please test things out. The last rebase unfortunately removed the three commit version and put everything into a single commit again. I'm worried I've rebased the wrong branch, but hope I've catched the correct state of the implementation.

Please let me know if you run into any issues.

[almereyda]

My intuition was correct. In trying to react quick with a rebase, I didn't fetch my force-pushed working branch from the other machine, which resulted in me pushing an outdated state of the PR.

Thank you for being so diligent in checking this out into an internal testing branch.

Please reset it hard by HEAD~1 and replace it with what is presented in:

• feat: add GitLab OAuth client #4828

🙈

[sriramveeraghanta]

Our team will take over from here. We will test it to ensure everything is working and address any issues that arise during the testing process.

[almereyda]

Thanks for the notice. Feel free to reuse, recommit or remix the content of the PRs in any way.

Also feel free to close #4828, if suitable.