don't let stringify convert `null` to `"null"`
compute UPTIME sensor
fix bug where nginx up-detection continued to run after service stoppage
remove the hand from sensor/config values, as they aren't clickable
use generics for RendererHint, simplify class hierarchy there, and add support for rendering based on class type, used for Duration and HostAndPort, and for Entity instances to show the name of the entity
This closes #208