Send OpenTSDB POST request as urlencoded

Update CHANGELOG.md

Fixed dashboard_tag query

Triggers grafana packer CircleCI build after successful build of master branch

Only triggers for main grafana repo (not forks) and when the
master branch is built.

Grafana stats view as mentioned in #3812

fix(graph): narrow panels shows correct date format on x-axis

fixes #3852. The function that calculates the date format for
the x-axis on a panel takes the panel width into account and
can be wrong for certain date ranges if the panel is too
narrow. E.g. can show dates in format %m/%d %H:%M when it
should show it as %H:%M

Serialize an elasticsearch error previously printed as 'Object'

Fixed broken tags search

Updates documentation for SingleStat thresholds

The implementation of thresholds changed after fixing #3248. Docs
updated to reflect and explain the change.

npm for node 4 on circleci is really old. This updates npm and will
hopefully make the build green again.

If OpenTSDB is 3rd-party hosted (or by another team) and does not support OPTIONS, send the request as POST (urlencoded)

Uses width set to 800px for invite div instead of position fixed. It
looks the same for desktop and makes the button clickable on smaller
resolutions and mobile. Although it is not really responsive so the
text will be small on mobiles. Better that though than a non-clickable
button.

After a refactoring the sidemenu-canvas css class disappeared so changed the code from #3741 to use the main-view class instead.