fix: sqlite3 and concurrent write requests caused database locked error some times, depending on how many concurrent requests and speed of disk drive. Fixed by reducign the number of concurrent db connections to 1 for sqlite3, fixes #7992

Merge branch 'heatmap-panel' of https://github.com/alexanderzobnin/grafana into alexanderzobnin-heatmap-panel

* support panel repeat for datasource template variable

* support All option

This seems to work for the three use cases:

- Overflow on the left
- Overflow on the right
- Elements that are out of view on page load

* use utf8mb4 character set for connections to mysql

* use utf8mb4 character set for tables, shorten varchar fields used in unique indexes

* migration type to update table character set

* update table character sets

* set charset for temp_user.status

* gofmt

Fix some spelling/grammar/wording problems in What's New in Grafana v4.0