Explore: Pluggable components from datasource plugins

JS tooling: run TS grunt tasks only when files changed

* Add new build info metrics that contains more info

The goal was to add more information about Grafana. But rather than
just adding those to the current metrics I created a new metric
since its a common pattern in the prometheus community to expose that
info in a metric named `*_build_info`.

We keep the old metric to avoid introducing any breaking changes but
we should be able to remove it the next breaking change

- using grunt-newer to prefix precommit tasks
- only got it to work for tslint and tsc

Not applied to:

- sasslint does not take the file arguments in a way that grunt-newer
  recognizes
- no-only-tests throws an error when used with `newer`, but it's
  sub-second runtime

Updated grafana_stats.json

Explore: fix copy/paste on table cells

When selecting text via mouse, our ReactTable cells' click handler
triggers an event.

- check event target to be the link, only then handle the event

- when instantiating a datasource, the datasource service checks if the
  plugin module exports Explore components, and if so, attaches them to
  the datasource
- Explore component makes all major internal pluggable from a datasource
  `exploreComponents` property
- Moved Prometheus query field to promehteus datasource and registered
  it as an exported Explore component
- Added new Start page for Explore, also exported from the datasource

Explore: error handling and time fixes

Revert to sync loading of css, sometimes js loaded before css which c…

build: adds branch info to binary build

Makefile: dependency-driven target to build node_modules

The grafana_stats.json used the following prometheus query: "increase(grafana_alerting_result_total[1m])" But a metric called "grafana_alerting_result_total" is currently not there anymore. So i changed the query to "increase(grafana_alerting_active_alerts[1m])" and updated the title as well (Before: "Grafana alert results", Now: "Grafana active alerts").

- added `node_modules` as new target
- dependency on `package.json` and `yarn.lock` allows for quick `make
  node_modules` after a branch change, which noops when the deps have
 not changed
- also added `clean` target

removes old invalid release guide

closes #13864

Merge branch 'fix-missing-points' of https://github.com/jsferrei/grafana into jsferrei-fix-missing-points

stackdriver: Remove redundant project name assignment

Fix label suggestions inside multi-line aggregation contexts

Implement sum aggregation suggestion with placeholder support

Fixing issue 13855

Related: #13615

Implements rudimentary support for placeholder values inside a string
with the `PlaceholdersBuffer` class. The latter helps the newly added
sum aggregation query suggestion to automatically focus on the label
so users can easily choose from the available typeahead options.

Related: #13615

Reduce re-renderings when changing view modes

This should cover use cases involving multi-line queries and range
vector syntax inside aggregation contexts.

Related: #12890