Add cancel_deadline field
Move signup_cancel_deadline value to cancel_deadline
Remove signup_cancel_deadline field
Refactor other usages of cancel_deadline field
Add spacing between operators
Group signup-methods together
Update comment
Refactor signup methods
Add comment
Handle null sentinel value in model for signup deadlines
Set signup_start to null if no start date
Let admin save no signup-start date
Default to the correct signup start date
Close signup when the start date has passed
Only display signup_deadline if earlier than start date
Merge pull request #26 from Turistforeningen/aktiviteter-signup-null

Aktiviteter signup null