Add page "Missing entries"

There should be an option to directly access a view with all missing lesson documentations (in the current school term, change possible).

added part::frontend prio::1 source::customer type::feature workflow::confirmed labels

created merge request !183 (merged) to address this issue

mentioned in merge request !183 (merged)

Since we already provide this functionality in a different view, we should make the list of missing entries the default in "My overview".

added board::todo label

added board::ready label and removed board::todo label

closed with merge request !183 (merged)

mentioned in commit e567ff54