Add gitlab as Oauth option to login and register accounts for cloudron gitlab instance and forum

[naDevi]

Cloudron use self-hosted gitlab instance but you dont offer gitlab login as Oauth to register or login to hosted instance and i think its possible to add for this forum.

The forum doesn't allow to submit that to suitable category (may be due new user?)

See https://git.cloudron.io/cloudron/box/-/issues/781

[girish]

@nadevi So, you are asking for a Sign in with gitlab.com button there ?

[girish]

I tried adding it now. I couldn't test it because my email id on gitlab.com is my cloudron email, so it's already taken.