Fixes: #9754

This PR adds severity enum (ok, warning, error) to health reports, and modifies rules based on which severity level is selected.

Rules:

• AccessURL: Warning if the endpoint is responding but it isn't HTTP 200
• Database: Warning if the latency threshold is breached, but database is responding
• DERP: Warning if one node in the region is unhealthy
• DERP: Use highest severity for the region if more than 1 node is unhealthy
• DERP: Warning if users must use WebSockets