* Refactor Explore query field

- extract typeahead field that only contains logic for the typeahead
  mechanics
- renamed QueryField to PromQueryField, a wrapper around TypeaheadField
  that deals with Prometheus-specific concepts
- PromQueryField creates a promql typeahead by providing the handlers
  for producing suggestions, and for applying suggestions
- The `refresher` promise is needed to trigger a render once an async
  action in the wrapper returns.

This is prep work for a composable query field to be used by Explore, as
well as editors in datasource plugins.

* Added typeahead handling tests

- extracted context-to-suggestion logic to make it testable
- kept DOM-dependent parts in main onTypeahead funtion

* simplified error handling in explore query field

* Refactor query suggestions

- use monaco's suggestion types (roughly), see
  https://github.com/Microsoft/monaco-editor/blob/f6fb545/monaco.d.ts#L4208
- suggest functions and metrics in empty field (ctrl+space)
- copy and expand prometheus function docs from prometheus datasource
  (will be migrated back to the datasource in the future)

* Added prop and state types, removed unused cwrp

* Split up suggestion processing for code readability

Add tslib to TS compiler

Karma to Jest: variable_srv_init

- using tslib reduces bundle sizes
- add compiler option for easier default imports of CJS modules
- remove double entry of fork-ts-checker-plugin
- speed up hot reload by using exprimental ts-loader API

Docs: Prometheus template query variables

Included example usages

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adjust table header contrast for the light theme

fix invalid reference

fix failing test due to time diff issues

Karma to Jest: graphite query_ctrl

Karma to Jest: completer

Karma to Jest: influx query_ctrl

docs: mentation that config changes requires restart.

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CloudWatch backend return 400 if user input error

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