change annotation refresher to use internal angular caching

[stuartnelson3]

the annotation refresher is doing it's own hacky thing to cache requests for annotations.

this can be replaced with https://docs.angularjs.org/api/ng/service/$http#caching, but maintaining the same api.

change the GET request to set cache: true:

$http.get(dashboardURL + 'annotations', {
  params: {
    'tags[]': t,
    until: until,
    range: r
  },
  cache: true
})

then have the clearCache function delete the $httpCache.

something along these lines. The rest of the code can go away.