settings(change): changed default value for allow sign up to false, closes #8743, and changed default option allow_org_create to false

If a dashboard has a null value for updated_by then the migration
for the dashboard version table fails. This change uses coalesce to
set -1 instead of null when inserting into the created_by column
which has a non-null constraint. Fixes #8783.

This reverts commit 8634c9d4.

Fixes #5356. Internet Explorer aggressively caches GET requests which
means that all API calls fetching data are cached. This fix adds a
Cache-Control header with the value no-cache to all GET requests to
the API.

* add alerting notifier: DingDing

* add alerting notifier: DingDing

* add dingding unit test

* add dingding unit test

* delete debug code & format code style.

* fix build failed: dingding_test.go

Fixes #8344. Documents the $__interval, $__interval_ms and $timeFilter
variables.

Adds docs for the new API endpoints, and for the dashboard history feature.

Merge branch 'refactor-basic-diff' of https://github.com/walmartlabs/grafana into walmartlabs-refactor-basic-diff

* dashboard: don't override timezone if default selected, issue #8503

* dashboard: hide UTC icon immediately after timezone changing

Fixes #8656 and properly sets the email_sent and email_sent_on fields
for a tempuser (signup user).

* heatmap: fix incorrect time for UTC timezone, fixes #8706

* heatmap: fix tests for time format

* histogram: don't cut negative values, issue #8628

* histogram: add percent/count option

* histogram: add tests for values normalizing

* histogram: improved ticks rendering

* histogram: fix default value in axes editor

Closes #8717