Restrict support requests to GitHub-connected users only

[jits]

Currently:

Support requests can be created by any logged in users. If they have a GitHub identity associated already then the support request issue created (on GitHub) will contain their GitHub username, and thus allow them to receive notifications. If not, then we use their hub user email address in the resultant GitHub issue.

Proposal:

After discussions with @oyelekci – we propose limiting support requests to only those users that have connected up their GitHub identity to the hub.

From a UX perspective, this means when someone lands on the support requests index page and does not have their GitHub identity connected up, then we should guide them to the the Connected Identities page to do so (and explain why they can't submit requests just yet).