Dashboard: promote features in the right bar

[humitos]

Small logic to decide which feature to promote.
This is pretty simple for now, but it shows the pattern we are thinking about.

Required by readthedocs/ext-theme#103

[agjohnson]

This is looking great. I know we'll come back to this and tune it, and would like to revisit it fairly often. I do see new users into an organization getting the security logs block initially, so the fallback is probably the first thing to change next.

I could also see us eventually matching new user onboarding content with our drip email content, and then going into this logic. All of this is good for next additions here though.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ UnboundLocalError: local variable 'template_name' referenced before assignment /dashboard/ View Issue
• ‼️ AttributeError: 'NoneType' object has no attribute 'pub_date' /dashboard/ View Issue

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