Fix "Recommended Actions" page

[vpetersson]

We added the page 'Recommended Actions' recently. It's currently just a place holder but will be important going forward. I haven't investigated it in detail yet but it throws a 500 when you try to load the page.

[a-martynovich]

@vpetersson The reason why it crashes is because actions.html requests "{% url 'action' 123 'xx' %}". This url is defined in backend/device_registry/urls.py only for MTLS role. That is, when it's a DASH role, it's not defined, and it's actually on a different port (and could be a different host too).

So we need to pass MTLS API URL to DASH somehow. What are your suggestions?