Offer backup creation in updater for 3.1

If updating a database from 3.0 to 3.1 breaks then unless users have backups they have lost their forum's data and it will be very hard to recover from - this is almost assured to happen to someone for some reason, whether it be due to an odd bug we haven't discovered or their error or a power cut during a migration (especially with soft delete which takes a long time on larger forums) or them wanting to then purge modifications from the database afterwards and the lack of a database cleaner for 3.1.

Can we please add a checkbox (ticked by default) to the updater[s] which will create a database backup when ticked before completing any database actions? We should also still advise them (boldly) to make their own backups however.

Logs from -dev:
<Marshalrusty> The 3.0 => 3.1 converter should encourage (read: really encourage) people to make a database backup before proceeding
<Marshalrusty> If something happens midway, there is no reverting like 2.0 => 3.0
<Marshalrusty> The ideal solution would be to make the backup for them
<MichaelC> Maybe like a checkbox which is ticked which makes a backup, if they untick it don't make a backup (As some people won't i.e. large sites)
<Marshalrusty> But at least should tell them "if you don't make a backup, be it at your own risk"