Official Github SAML support

Created by: emchap

Feature request description

A way to directly log in from an IdP to Sourcegraph when syncing permissions from GitHub.

Is your feature request related to a problem? If so, please describe.

Currently we require a two-step process—log in to the code host using SAML, then use OAuth from the code host to Sourcegraph. Doable, but not at parity with our Bitbucket and Gitlab experiences.

Describe alternatives you've considered.

The current workflow documented above.

Additional context

Documented after conversation with @unknwon