Query field refactorings to support external plugins

* make diff and percent_diff tests more realistic

* fix diff and percent_diff

* include @marefr's additional tests

* tabs to spaces

testing commit permisions :)

* revert

* Update lodash and moment

* tabs to spaces

testing commit permisions :)

* revert

* tabs to spaces

ux: minor fix/tweak to inactive view mode, think logo should be visible & fixes dashboard title alignment

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Fix datasource sorting with template variables

- fixes sorting when template variables are present
- simplified existing test cases
- added test to cover variable usage in datasource service

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Prometheus heatmap: fix unhandled error when some points are missing

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Fix links not updating after changing variables

mysql,mssql: improve $__timeFilter, $__timeFrom, $__timeTo macros

The $__timeFilter macro now uses a BETWEEN with timestamps instead of 
comparing epochs
The $__timeFrom and $__timeTo macros now return strings of format 
YYYY-MM-DDThh:mm:ssZ instead of epoch

Karma to Jest: elasticsearch datasource

Fix HTTP-API admin doc

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