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