Fix naming scheme for role types

[naglepuff]

In order to work with permissions in PermissionDialog.vue, there are two declared types in utils/permissions.ts, being Role and SingularRole. Further, there exists an interface called UserPermissionSpec with two properties: role: Role and user: UserSpec.

Semantically, I'd argue that an object which looks like {role: 'writers', user: {...}} makes little sense, since the actual role of this user would be writer. The type Role should be singular, and we should have a new type along the lines of PluralRole. Furthermore, the function names of singularRoleToPlural(role: SingularRole) and pluralRoleToSingular(role: Role) should be changed to singularize(role: PluralRole) and pluralize(role: Role). The typing of the function signatures already communicates what is being changed.

This is not a high priority set of changes, but they would make our code easier to read.

[waxlamp]

For the record, naglepuff and I discussed this change and the ideas expressed are a mix of ideas that came out of our meeting. @AlmightyYakob if you have thoughts on this, please share.