* updated rules for variable name and fixed files that didn't follow new rules
* fixed test so it uses new rule
* made exceptions to rule in interval_variable
* graph: initial histogram support #600
* graph histogram mode: add Bars number option
* graph histogram mode: fix X axis ticks calculation
* graph histogram mode: change bar style (align and width)
* refactor(graph): move histogram functions into separate module
* graph histogram mode: rename series to "count"
* graph histogram mode: fix errors if no data
* refactor(graph and heatmap): move shared code into app/core
* graph: add tests for histogram mode