After support for queries in template variables was added to InfluxDB,
it can be necessary to added dymanic time constraints. This can now be
done changing the variable refresh to "On Time Range Changed" for
InfluxDB

Make prometheus value formatting more robust

- In 0425b477 the labels suggestions were refactored and a typo broke
  the look up for available lables of a metric

Use url params for explore state

Dont parse empty explore state from url

* get id from path param when updating a playlist

* add playlist http api documentation

* remove required condition for the id in update cmd

Explore: calculate query interval based on available width

- classic dashboard panels inject a dynamic query interval as part of
  the query options. Explore did not have that.
- this PR adds the interval calculation to Explore
- interval based on Explore container's width
- ensure min interval if set in datasource

- putting state in the path components led to 400 on reload
- use `/explore?state=JSON` instead

- only parse url state if there is any
- prevents parse exception in the console on empty explore state

Adding eval_data to alerts query results

Adds commented out settings in nginx conf
for testing basic auth and auth proxy

- prometheus datasources passes its own interpolator function to the
  template server
- that function relies on incoming values being strings
- some template variables may be non-strings, e.g., `__interval_ms`,
  which throws an error

This PR makes this more robust.

* devenv: working on dev env setup & dashboards

* devenv: refactored testdata app to a built in datasource instead, and moved dashboards to a devenv provisioned dashboards

Explore Datasource selector

Author:    Mark Meyer <mark@ofosos.org>

* Comment out unused vars

* Remove unused sass vars

Adds a datasource selector to the Explore UI. Only datasource plugins
that have `explore: true` in their `plugin.json` can be selected.

- adds datasource selector (based on react-select) to explore UI
- adds getExploreSources to datasource service
- new `explore` flag in datasource plugins model
- Prometheus plugin enabled explore