Add GitLab Social Authentication

[jefmoura]

Feature Request

As a user of Buildly Core, I'd like to have the option to also log into it using a GitLab account.

Proposal

Buildly Core already implements GitHub and Google social authentication and we'd like to extend the support to GitLab.
We need to provide some configuration process as we do for GitHub and Google, so GitLab can also be used for authentication.
We are using the package python-social-auth to implement all social auth schemas and it also provides GitLab as we can see here.