Check DB privileges

Original reported by @kehugter in https://github.com/nextcloud/server/issues/2870

I keep my DB user with minimum privileges during normal usage for security reasons. The update using the updater app failed because of missing DROP and INDEX privileges.

Steps to reproduce

1. Update using the updater app. Finish the first part successfully
2. Return to the main site to finish updating the database
3. Update fails because of missing DB privilleges

Expected behaviour

During the sanity check, the user is warned about insufficient DB privileges.

Actual behaviour

No DB privileges check is done, user gets an error message after the Nextcould instance is unusable.

Server configuration

Operating system:
Debian 8.6
Web server:
nginx
Database:
mysql
PHP version:
5.6.29
Nextcloud version: (see Nextcloud admin page)
10.0.2
Updated from an older Nextcloud/ownCloud or fresh install:
Updated from Nextcloud 10.0.1
Where did you install Nextcloud from:
tar archive

Start by tracing the updater's sanity-check and database-update flow, then reproduce the failure with a database user lacking DROP and INDEX privileges. Done means the sanity check detects insufficient privileges and warns before the update can leave the Nextcloud instance unusable.