Markdown and learning progress features are no longer considered
experimental. Leaning progress can no longer be globally disabled.

Added 'ui' prefix for splash screen settings.

An LDAP search pattern can now be used instead of binding with a DN
pattern.

This reverts commit fc78eb18.

SASS variables only exist at compile time and are not available for
arsnova.properties.

Renamed optional.arsnova to ui.arsnova-slogan and made comments start
with an uppercase letter.