Add nextcloud provider

[Ramblurr]

Description

This PR adds the Nextcloud provider, that allows for authentication of users on a Nextcloud instance.

Motivation and Context

For some time now Nextcloud has had an Oauth2 implementation that allows for arbitrary 3rd party tools to act as oauth2 clients and perform authentication for users hosted on the Nextcloud instance.

Docs: https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/oauth2.html

How Has This Been Tested?

• I've included a test suite
• I'm running this in production on a large Nextcloud instance

Checklist

• My change requires a change to the documentation or CHANGELOG.
• I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.

[JoelSpeed]

@Ramblurr Thanks for adding this support. I'm happy for us to support this but as I have no way to test this, could you please add yourself to the CODEOWNERS file taking ownership of the relevant files. This way if there are any issues with it, people should be able to ping you?

[Ramblurr]

Thanks for the review. I'm leaving for vacation soon, so I'll get back to this in several weeks.

[Ramblurr]

I'm back at this again, sorry for the delay @JoelSpeed.

I rebased and patched my PR given the recent shakeup for 4.0.0, and I've made the requested changes.

The Travis build is failing due to a linting error in code I didn't touch:

GO111MODULE=on "/home/travis/gopath/bin/golangci-lint" run
providers/azure_test.go:116:47: string `Bearer imaginary_access_token` has 3 occurrences, make it a constant (goconst)

			} else if r.Header.Get("Authorization") != "Bearer imaginary_access_token" {
			                                           ^
Makefile:20: recipe for target 'lint' failed
make: *** [lint] Error 1
The command "./configure && make test" exited with 2.

This is code in master already.

If I disable linting and run just the tests, they pass fine locally.

[steakunderscore]

} else if r.Header.Get("Authorization") != "Bearer imaginary_access_token" {
This came up somewhere else in the repo. Just change the string so it's unique. Could use something like azure_imaginary_access_token. I think this should fix it.

[Ramblurr]

Should I add that as a commit to this PR?

[ploxiln]

In the function testNextcloudBackend() you added the line:

			} else if r.Header.Get("Authorization") != "Bearer imaginary_access_token" {

and that is what caused this dumb linter rule to be triggered. So to quiet the rule, you can change your tests to use a different mock access token string.

[Ramblurr]

Oh! I see, the linter is comparing the strings across the entire project. Sorry about that. That explains why this isn't present on master :)

Pushed the fix.

[loshz]

Added a few tiny nits (sorry).

Are you ok to add yourself to the CODEOWNERS for this provider? Totally optional of course 😄

[Ramblurr]

Added a few tiny nits (sorry).

fixed!

Are you ok to add yourself to the CODEOWNERS for this provider? Totally optional of course 😄

If you're referring to .github/CODEOWNERS, then I've already added myself there :)

[loshz]

LGTM 👍

[loshz]

This has been open for a while so let's get the changelog conflicts resolved and we can merge.

@JoelSpeed: if you've time to re-review then that would be great!

[Ramblurr]

Fixed the CHANGELOG conflict.

[loshz]

Thank you and congrats on your first contribution! 🎉

[JoelSpeed]

One quick question about the docs, I may not be understanding fully though

[JoelSpeed]

Should this be gitlab or nextcloud here? Looks like maybe a copy paste error?

[Ramblurr]

Woops, you are right. Should definitely be nextcloud.

[Ramblurr]

Fixed doc typo and rebased master.

[JoelSpeed]

The changelog entry is in the wrong place right now but I'll fix that up later, thanks @Ramblurr for being patient with us!