Default login page for Jetstream2

Problem/Opportunity Statement

Jetstream2 will use OpenID Connect login via Globus using XSEDE credentials.

What would success / a fix look like?

• Default login option on Jetstream2 site should be XSEDE credentials. Don't just redirect people there but show the button foremost.
• Secondary login should be via OpenStack account, for folks who want to add another cloud to the UI.
• Jetstream1 login should go away. This means we need to make it optional via configuration, then remove it from the codebase entirely. Turned this into #654 (closed)