Sort teacher abbreviations alphabetically

The teacher abbreviations in the substitution plan header are not sorted. The alphabetical sorting makes it easier to enter them quickly.

added part::frontend label

assigned to @hansegucker

added source::customer type::feature workflow::confirmed labels

created merge request !109 (merged) to address this issue

mentioned in merge request !109 (merged)

closed with merge request !109 (merged)

mentioned in commit 2bab9c2c