Allow deletion of admissions from GUI

[norbye]

Admissions can be deleted by any member of Webkom or by the creator of the admission if the creator is staff (revue boss, hs big boss, backup boss) AFTER the admission has closed. (aka the same people that have permission to edit the admission)

Changes;

• Write tests to cover deletion
• Add button in admin "manage admission" view
• Add backend functionality to delete

Minor stuff that bugged me along the way;

• Admissions created using python manage.py create_admission were missing slugs, now they don't
• The modal component had no way to customize the action buttons, now it does

Before;

After;

When it's not possible to delete it

When it is possible

Confirmation dialog

Resolves ABA-410

[linear]

ABA-410 Allow deleting admission from adminpanel after it is closed

[ivarnakken]

Also much needed!

I much like the safeguard of not being able to delete until it has closed