feat(backend): add gitea backend

[denyskon]

Summary

Introduces Gitea as new backend for Decap CMS. Fixes #1109

Checklist

• added backend
• added unit tests
• added docs
• tested creating/updating/deleting entries, upload/delete media

A picture of a cute animal (not mandatory but encouraged)

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[denyskon]

This is very WIP, I neither tested it enough nor did I ensure I did not break tests. I will notify when the PR is ready, for now it's just for the ones who're curious if there's something happening.

[denyskon]

@martinjagodic I resolved the conflicts and changed the naming to match the recent changes. One question, as this is my first time working with such a monorepo: do I have to bump the version number for the decap-cms-lib-auth package as I changed it too?
Besides from that, this PR should be ready again.

[martinjagodic]

Thanks, @denyskon. Versions are bumped automatically when we create a release, so you don't need to do that.

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[martinjagodic]

Hi @denyskon I know it's been a while... Are you willing to dive into why tests are failing?

[denyskon]

@martinjagodic Sorry, I didn't notice the lint failure, and build failed because of a dependency update on master. It should all pass now :)

[demshy]

Great! We could add some e2e tests, but since they are probably awaiting a huge overhaul I think we can add that later.