Create model for application state

[Snorre98]

Look into if recruiter_priority, recruiter_status, and application_state should be in their own model (ex. RecruitmentApplicationState), with related methodes.

Right now they are implemented in the RecruitmentApplication model, so we would need a relation between the mentioned model (ex. RecruitmentApplicationState) and RecruitmentApplications.

As recruiter_priority, recruiter_status, and application_state are fields with data not provided by the applicant when applying I feel that it makes sense to have them in a separate table.
This might also make it easier to expand on the application state functionality, like mentioned in #1577

❗ this issue is related to #1575 and #1577. If they are solved prior to this the soultions for those might have to be taken into consideration here.