1. Addition of external source icons for services marked as such.
2. New %with-tooltip css component (wip)
3. New 'no healthcheck' icon as external sources might not have
healthchecks
4. If a service doesn't have healthchecks, then the [Health Checks] tab in
the Service detail page is hidden, we use the [Services] tabs as the
default instead
5. `css-var` helper. The idea here is that it will eventually be
replaced with pure css custom properties instead of having to use JS. It
would be nice to be able to build the css variables into the JS at build
time (you'd probably still want to specify in config which variables you
wanted available in JS), but that's possible future work.

Lastly there is probably a tiny bit more testing edits here than usual,
I noticed that there was an area where the dynamic mocking wasn't
happening, it was just using the mocks from consul-api-double, the mocks
I was 'dynamically' setting happened to be the same as the ones in
consul-api-double. I've fixed this here also but it wasn't effecting
anything until actually made certain values dynamic.

When there are no checks, still show the healthchecks tab in the node
detail page, but default to the Services tab. The healthcheck tab panel
now shows a 'no healthchecks' message

It's now only a tooltip because the design says so, which I would guess
would make for a better a11y experience