login_context now gets the social_backends list through
get_social_backend_dicts and we move display_logo customization
to backend class definition.
This prepares for easily adding multiple IdP support in SAML
authentication - there will be a social_backend dict for each configured
IdP, also allowing display_name and icon customization per IdP.
There are a few outstanding issues that we expect to resolve beforce
including this in a release, but this is good checkpoint to merge.
This PR is a collaboration with Tim Abbott.
Fixes#716.
Adblock Plus's "Block social media icons tracking" setting blocks
images with for social media platforms in their names from loading, so
we rename the Google logo to bypass this.