fixes #520 , adds worker assigned (if any) to the tooltip information in appointment calendar

Closes #520

See merge request !434

this changes prevents trying to replace placeholders not used in the file....

See merge request !431

Fix/502 - vouchers hours can be negative value

Closes #502

See merge request !435

Fix/513 - change import on export

Closes #513

See merge request !436

removes alpha from title

Closes #521

See merge request !433

Feature room planning

See merge request !430

this changes prevents trying to replace placeholders not used in the file. However, if a field value is None it will still fail. We need to discuss the approach to follow if a required field is none, should it be replaced by the text None or how can we propagate an error to the frontend?

subject_type was not retrieved from the given ID in the URL. fixes #516

Closes #516

See merge request !429

added placeholder and warning if no date is provided on recurring events

See merge request !428

change mirror address to the new one

See merge request !427