Multiple IdPs for SAML

Blog: https://about.gitlab.com/2016/03/30/feature-highlight-saml/#comment-2600200692

Description

Add support for multiple IdPs with SAML.

Concerns

From a brief discussion with @patricio the following points came up:

• We currently don't have plan for it.
• Not sure how possible that would be.
• It might need to be implemented in the omniauth-saml side first.

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/cc @patricio