Improve the formatting of the substitution plan

The margins should be narrower and the box for absentees should be displayed as a table.

assigned to @fph

created merge request !206 (merged) to address this issue

mentioned in merge request !206 (merged)

added part::frontend source::customer type::bug labels

removed type::bug label

closed with merge request !206 (merged)

mentioned in commit f1f8b712

unassigned @fph