Draft: Support GitLab

[Yoric]

Untested so far, for some reason webext seems broken for me.

[mikeconley]

I had to upgrade my copy of web-ext like this:

npm install --global web-ext

and then I could load it (I ended up removing the --pre-install change from the package.json file).

I'll say that your patch is definitely on the right track - you've added the right things in the right places. I'm glad my circuitous, hastily-written code is still somewhat serviceable! :)

I, unfortunately, don't have a GitLab account anywhere to test this on, so I might have to rely on you for that.