Add Mattermost authentication strategy

[DoubleMalt]

I added the possibility to authenticate against a Mattermost server. I was not sure about the version numbers, but I'm happy to change them to what they should be.

[SISheogorath]

Awesome PR! Instantly coming with all needed changes. Very nice!

But as you see the CI process fails because of not following the coding standard. Can you fix this, please? Only minor issues ^^

You can test it yourself on your dev machine using npm test

Thanks a lot~

[DoubleMalt]

Done.

[SISheogorath]

Could you squash the commits using git rebase -i HEAD~6 so we don't have to many "Fix" commits in our git history?

Also one important question: Is this new auth provider tested? Because I'm not really sure how to test it since I don't run a mattermost instance myself.

Oh and please since the contributor license agreement #578 because we'll change the license soon to AGPL.

[DoubleMalt]

Squashed.

The new authentication mechanism is used at the moment for https://notes.doublemalt.net. I can create a test team there so someone can test it against it if desired.

Agreed to the license change.

[SISheogorath]

When you run it (in production), that's fine for me 😄

Only want to prevent untested code.

We have to keep an eye on FortAwesome/Font-Awesome#9727

Currently, there is no mattermost icon :/

But a missing icon will not prevent us from having this awesome feature!

Thanks a lot!