Add user roles for organizations/gangs/sections

[robines]

This PR introduces a new custom Auth backend. It adds a new Role model, which basically works as a grouping of permissions. It also adds a user's connection to these Roles. This must be done on one of the following hierarchical levels: Organization, Gang, Gang Section. This is also the order the new auth backend checks for permissions.

The auth backend relies on the has_perm method from Django, and for our backend, the obj argument is required. This means the new backend only checks permissions against specific provided objects.

For each hierarchy level, it resolves the organization, gang or section, and returns True if the user has the required permissions on that level.

Organization/Gang/Section resolvers

The way the backend knows what organization an object belongs to is through the new resolve_org/resolve_gang/resolve_section methods. The purpose of these methods for an object is to return the org/gang/section which the object belongs to. For instance, a Recruitment is owned by an organization, therefore resolve_org returns that organization.

Not all objects have a single owner, if any at all. In that case, they simply don't implement the resolvers.

The resolvers currently have a single argument: return_id. The purpose of this argument is to avoid having to unnecessarily fetch a whole instance from the database, when we only need the ID. All models which implement the resolvers must respect this argument if possible.

The great thing about the introduction of these resolvers is that they may be used everywhere, not just in the new auth backend! It greatly simplifies code, and makes it a breeze to get the org/gang/section owner of a specific object.

Example

For example's sake, say we have a "Recruitment Admin" role. It contains all relevant recruitment permissions. This role may be given to users on either the org/gang/section level (or multiple levels). If the user has the role on the Organization level, they receive the permissions for the selected organization, as well as for all the organization's gangs and roles. This means they can manage every recruitment object which belongs to the organization.

Similarly, if the user has the role on the Gang level, they receive the permissions for the selected gang, as well as for all the gang's sections. They can then manage all recruitment objects which belongs to the gang.

[magsyg]

Ser ok ut
Men sett opp tester for hver av typene
Typ hva om man har/ikke kan gjøre noe basert på org/gang/generelt

[magsyg]

flere tester

[Snorre98]

lgtm

[Mathias-a]

Har ikke slettet eksisterende funksjonalitet?

[robines]

@Mathias-a Føler det faller litt utenfor scopet til denne PRen, siden det kreves ikke for å få dette rollesystemet til å funke. Så har laget en ny issue på det #1342

[magsyg]

Looks good