Prevent lab creation without assignments and collapse lab create buttons into one

CHANGELOG.md

...n/java/nl/tudelft/queue/controller/EditionController.java

src/main/resources/templates/edition/view.html

src/main/resources/templates/edition/view/labs.html

src/main/resources/templates/lab/create.html

src/main/resources/templates/lab/create/capacity.html

...sources/templates/lab/create/components/advanced/lab.html

...ces/templates/lab/create/components/advanced/session.html

...esources/templates/lab/create/components/lab-general.html

...rces/templates/lab/create/components/session-general.html

src/main/resources/templates/lab/create/exam.html

src/main/resources/templates/lab/create/regular.html

src/main/resources/templates/lab/create/slotted.html