Prometheus heatmap: fix unhandled error when some points are missing

[skip ci]

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

[skip ci]

[skip ci]

[skip ci]

[skip ci]

[skip ci]

[skip ci]

Currently if a null appears in a table column, for instance in data
returned by postgres, sorting on that gives an arbitrary order. This
is due to null being neither greater or less than any string, which
makes the sort unstable.

Change the table sort function to compare on nullness first. Note
this is a slight behaviour change for numbers, which would otherwise
treat null and 0 as equivalent.

Signed-off-by: Martin Packman <gzlist@googlemail.com>

* Fix bar width issue in aligned prometheus queries

This was broken because null values were filled in with unaligned times.

* use aligned times for result transformation
* add tests

An earlier version of this fix aligned the times again in the transformer, but
I think it's safe to only deal with aligned times in the response.

* Fixed prometheus heatmap tranformer test

The interval needs to be 1 to prevent step alignment.

foo.bar.com in description, but foo.bar in configuration.
Updated description, makes more sense then changing the configuration :)

* Karma to Jest: datasource_srv

* Sort function differs between Karma and Jest

* Fix error based on .sort() implementation

* Remove Karma test

* alerting: only log when screenshot been uploaded

* Remove comments

* Karma to Jest: datasource_srv

* Sort function differs between Karma and Jest

* Fix error based on .sort() implementation

* Remove Karma test

* Remove comments

* Remove console.log

* Remove console.log

* Change sorting and add test for default data source