Add option to disable/enable Q&A to settings

Settings:

If Q&A is disabled, related settings are disabled as well.

The SettingsSlideComponent has been extended to be able to disable the slide toggle.

Q&A:

Disable and hide functionality and show a hint if comments are disabled.

Furthermore let creators activate the comments feature from here.

Navigation: Hide comment navigation for participants if feature is disabled.

Closes #998 (closed)