Support self-hosted gitlab

[vovsemenv]

First of all - thanks for the amazing work. This is much better then the original gitlab diff viewer.

but this is supports only cloud gitlab by default. for example our company self-hosted https://gitlab.qatools.com will not match

      "matches": ["https://gitlab.com/*"],

[Stanzilla]

The problem will be page permissions, I kinda do not want to request permission for every domain