Issue #818: Add option to enable / disable auto-updater

[Sebbo94BY]

Adds an option in the webinterface to enable and disable the auto-updater feature:

Reasons, why people / administrators want to disable the automatic update and do it rather manually instead:

• People don't want, that third-party tools call home on a regular basis
• Allows testings of newer versions in a test environment
• I personally usually always wait 1-2 weeks after the release with the upgrade in order to see, if other people open any bug reports or report issues
• Ability to plan and announce a maintenance for the update incl. database and file backups etc.
• Ability to update the application using Git to a specific tag (instead of simply overwriting all files with a ZIP file)
• Ability to immediately fix issues, if the update should fail (this is not possible, if the auto-update does this when I'm sleeping or on vacation)

Solves #818