Allow Gitlab user to use on premise instances of gitlab as oAuth provider

**Why ?**
In order to let you access a project on Commentor, we check with your oAuth provider if you still have access to this project. Then this means that you wont be able to use Commentor today with an auto hosted instance of Gitlab as we will check if you have access to it through gitlab.com who doesn't know this.

**What ?**
Whenever a user hits the connect with gitlab button

We should present him a modal with a "Gitlab.com" Button and an input with a placeholder saying "Specify your on premise gitlab instance" and a button trigering the connexion to this instance.

This means we should also store in the cookies the gitlab instance this user wants to be connected to and use it instead of gitlab.com on the calls from the Gitlab oAuth provider