Allow admins to remove the `Login with GitHub` button, while still allowing for import to work

https://gitlab.zendesk.com/agent/tickets/15965

If admins want to enable Import from GitHub, they need to add the Omniauth settings to GitLab's configuration, but in doing os they also enable login via GitHub:

This integration is required to enable connection a gitlab account with a github account, so that the import has the required context and permissions to load github data.

As a consequence, however, there is a github login icon in the login page. This is not necessarily desired. In our case it is not desired at all: We want power users to be able to connect their accounts for import purposes, but we would like to avoid people using github for login-integration.

As the customer mentions, Login with GitHub is not something you might to be always enabled. A new config setting could solve this, something like login_button: false and make it default to true.

This same thinking would apply to BitBucket as well.

@JobV what do you think?

/cc @DouweM