Merge branch 'uid-migration' into 'master'

Add migration which converts database and LDAP UIDs to lowercase

ARSnova Backend now converts all user IDs for database user accounts to
lowercase. This migration converts existing accounts for compatibility.
Documents mapped to LDAP UIDs are also reassigned to lowercase UIDs
since most LDAP configurations use case-insesitive comparison for UIDs.

This change was done to prevent duplicate account creation.

See merge request !3