Implements rudimentary support for placeholder values inside a string
with the `PlaceholdersBuffer` class. The latter helps the newly added
sum aggregation query suggestion to automatically focus on the label
so users can easily choose from the available typeahead options.

Related: #13615

Explore: move suggestions logic to datasource language provider

[skip ci]

mysql: fix timeFilter macro should respect local time zone

use default region to call DescribeRegions

support template variable in stat field

docs: improve ES provisioning examples

Fix megacheck issues

closes #12281

[skip ci]

fix panel solo size

Fix doc error for DingDIng alert

Update the regex-matching in templateSrv to work with the new variabl…

Fix cannot receive dingding alert when alert message is empty

[skip ci]

[skip ci]

[skip ci]

Delete provisioning meta data when deleting Folder

prior to this fix Grafana didnt delete meta data
about the provisioned dashboard in `dashboard_provisioning`
which means that the dashboard wasn't inserted into
Grafana again if the folder was delete within Grafana.

closes #13280

Update the regex-matching in templateSrv to work with the new variable-syntax and be more flexible to regex-changes #13804

Explore: fix graph resize on window resize

[skip ci]

[skip ci]