Add checkbox preference to perform full database dump before performing a upgrade

[Flowdalic]

Would be nice if piwik's upgrade dialog would provide a checkbox preference that allows user to optionally perform a full database dump before the upgrade is performed.

Since sometimes the automatic upgrade process fails for some reason (see for example #6280, and some users (including me) don't do (for some mysterious reasons) automated database backups. So it would be nice be conveniently perform those just before an upgrade.

[mattab]

Thanks for the suggestion. Unfortunately we won't provide this feature in the Piwik core itself as creating a DB dump can take along time and would be error prone. Though it would be a nice feature done as a plugin. Cheers!