Add support for Gitea OAuth

[KNOXDEV]

As long as this is a project for self-hosted open directories, we might as well add support for self-hosted github-like services like Gitea: https://docs.gitea.io/en-us/oauth2-provider/

Shouldn't require anything complex on top of what the Github OAuth already does other than the ability to configure the Gitea instance hostname in config.json, which is arguably what the Github configuration should already have since self-hosted Github Enterprise is a thing.

[KNOXDEV]

Just briefly glancing at the project structure, it seems like OAuth is provided by your external library, https://github.com/nektro/go.oauth2

There's nothing wrong with this, but the project doesn't seem to have support for configuring providers with a custom hostname, which means adding this kind of support isn't 100% drop in addable without a few structure changes. In other words, it might be involved.

[nektro]

Since GitTea is a self-hosted service it cannot be added to nektro/go.oauth2 as a pre-loaded identity provider, but I also just created #34 which should solve this issue and support more self-hosted services in the future.

[nektro]

A section will be needed to be added to config.json that allows users to load their own Providers at outlined in https://github.com/nektro/go.oauth2/blob/master/provider.go

[KNOXDEV]

Alright, so long as there's a solution.