Add more granular permission sets

[bntrtm]

Pincher needs to offer users for budget admins and managers further control over how contributors may contribute, and what viewers may view.

Consider the following examples:

1. You're a parent who wants to add their child to the budget with the VIEWER role, to see their allowance. Without more granular settings on your child's viewership permissions, they can see everything about the family financial situation, and for one reason or another, that may not be ideal.
2. You're a small business with a few employees added to the budget with CONTRIBUTOR roles. They should be able to log transactions of course, so that your accountant has a solid history of expenses charged to company cards. But while these employees should be able to update these transactions within a particular frame of time in order to correct clerical errors, their ability to do so ought to be constrained to a timer.

New user membership features should be implemented that allow for members of the ADMIN and MANAGER roles to define custom permission sets assignable to different users of varying roles in a given budget.