Add fill mode to fill in previously seen value when point is missing to SQL datasources

Fixing bug in url query reader and added test cases

[skip ci]

Fix issue with secret fields after updating datasource

fix missing *

The missing * causes the text to be in the box to be displayed incorrectly.

Explore: prometheus query helpers 

- use color variables for cascader styles
- fix Table value type

[skip ci]

WIP: Fit panels to screen height

- load all histogrammable metrics on start, based on `{le!=''}` series
  query
- select dropdown shows all those metrics as well as histograms in a
  special group
- select a metric will write fill the metric in the query box
- if a histogram is chosen, it will write a complete
  `histogram_quantile` query
- added new dependency: rc-cascader

- clears the content of a query row

- Clear All button to clear all queries and results
- moved result viewer buttons below query rows to make it more clear
  that they govern result options

Explore: show message if queries did not return data

- every result viewer displays a message that it received an empty data
  set

- position was determined by SPACE, but Prometheus selectors can
  contain spaces
- added negative lookahead to check if space is outside a selector
- moved braces plugin into PromQueryField since braces are prom specific

Explore: Add history to query fields

* build: makes the grafana docker image smaller.

* build: branches and PR:s builds the docker image.

Mssql: add logo

unix socket docs

- _.reverse() was modifying state.history

- queries are saved to localstorage history array
- one history per datasource type (plugin ID)
- 100 items kept with timestamps
- history suggestions can be pulled up with Ctrl-SPACE

Merges the Grafana docker build into the main repository.

* Explore: facetting for label completion

- unified metric and non-metric label completion
- label keys and values are now fetched fresh for each valid selector
- complete selector means only values are suggested that are supported
  by the selector
- properly implemented metric lookup for selectors (until the first
  metric was used which breaks when multiple metrics are present)
- typeahead tests now need a valid selection to demark the cursor

* Fix facetting queries for empty selector

Karma to Jest: variable_srv