Optimized prosody configuration for full conversations.im feature compartibility including full OMEMO support

- fixed backups not downloadable on multiserver slave, fixes #4973

- fixed accidently deleting mysql r/o user completely when it is removed from a single database, fixes #4963

Fix bug where in a multiserver setup, deleting a backup will not remove its details from the master database as well.

Fix typo in parent user related variable when creating new jailkit shell user. This bug was interfering with the correct functioning of the plugin.

+ fix php-fpm chroot

Fix #4808 Load the existing keys without new line, so array_search can find the old ones. Unset a key from existing_keys only if a match is false and avoid false being treated as 0

Fixed a problem that caused the creation of mysql users to fail. This affected the dev branch only and not the stable 3.1.6 release.

Change check order for welcome-email template. First it should use specific welcome messages for the domain (html before text) and than language specific templates. Otherwise it would use a german html language file before a domain specific text file if html file does not exist.