Do not expose resolved alerts & do not send resolved if never firing

[mxinden]

Do not expose resolved alerts on the /alerts endpoint. Do not send
resolved alerts to receivers if the alerts have never been fired before.

This is related to #815, but does not resolve the specified issue.

[juliusv]

Travis is failing with:

/app/ui/app/tests/Main.elm has an invalid module declaration. Check the first line of the file and make sure it has a valid module declaration there!
Makefile:6: recipe for target 'test' failed
make: *** [test] Error 1

[juliusv]

I can confirm that this fixes the UI issues I observed in #815

[stuartnelson3]

Try rebasing against master, @w0rm just updated our tests to run on CI

[w0rm]

@stuartnelson3 the problem is because https://github.com/prometheus/alertmanager/blob/master/ui/Dockerfile doesn't fix the versions of elm-format and elm-test, it should be elm-format@0.6.1-alpha and elm-test@0.18.3.

[mxinden]

Rebased PR. Tests are all green.