Explore: reset typeahead on cursor move

Explore: fix rate hint for series with null values

Moving contributing.md to top level

Explore: typeahead and render performance improvements

- remove `edited` property of query objects
- query object semantics changes to be initial values only
- resetting or externally modifying a query is done via giving it a new
  react key, which replaces the row completely
- query changes while typing are sent up to Explore to be saved in a
  local property that is not part of the component state (prevent
  control circle)
- all Explore functions using the queries must access the local property
  for current values

- Lower debounce
- refactor suggestion selection
- make Portal pure
- use prev suggestions object if suggestions are the same

move timeFrom, timeTo, unixEpochFrom and unixEpochTo macros to sql_engine

[skip ci]

Keep trailing slash for datasource proxy requests

Fix megacheck issues

centered dashboard icon in search with flexbox

stackdriver: adds missing nginject attribute

Fix issue with updating role permissions #13507

build: automatically publish releases to grafana.com.