Fix typo in error message from update checker

Closes #4923

Add documentation on the build system for occasional contributors

fix(singlestat gauge): better font size handling for gauge, now value font size is used as scaling factor, fixes #4876

fix broken link for kairosdb image

onDataError was calling onDataReceived with {data : []} which breaks at the first line since object has no map function.
The correct form is probably just []

Bring back threshold line mode

* removing useless code

* fix various minor typo

* don't silence phantomjs timeout and allow more time

* fix gofmt

Threshold line mode was somehow discarded from the HTML in the ui redesign, this brings it back

Wrapped the form in a form tag and added a submit, the ng-click shoudl work the same, but now enter works. @torkelo, pls review (#4898)

Add more documentation on how to fork the project, and how to
build on changes using grunt