Merge branch '345-adding-absences-using-absence-creation-dialog-doesn-t-work-within-a-day' into 'master'

Resolve "Adding absences using absence creation dialog doesn't work within a day"

Closes #345

See merge request !455

Resolve "Room substitutions are not shown correctly"

Closes #343

See merge request !454

Merge branch '341-absence-creation-dialog-doesn-t-handle-min-max-date-times-correctly' into 'master'

Resolve "Absence creation dialog doesn't handle min/max date times correctly"

Closes #341

See merge request !446

Re-set start datetime when end datetime gets set to an earlier value (and vice versa) in absence creation dialog

Update pyproject.toml

See merge request !354

Update .dev-js/package.json

See merge request !448

Update tox.ini

See merge request !447

Do release 4.0.0.dev9

See merge request !449

Resolve "Add export functionality to course book"

Closes #258

See merge request !422

This just looks more uniform.