Dashboard fails to load if MITXONLINE_URL is not set

[pdpinch]

Since we have MITX_ONLINE set in all our deployed environments, it's not a critical issue. But it will eventually confuse some developer running the site locally without a value.

Steps to Reproduce

1. Don't provide a value for the new setting MITXONLINE_URL
2. Load /dashboard

Expected Behavior

• dashboard loads
• If the user should be given a message that contains MITXONLINE_URL the link should be omitted.

Actual Behavior

• Most of the page does not load due to a JavaScript error