Fix Users App Permissions

While there are quite a few permissions in place for the Users app, there some issues with them:

• Many permissions confer access to more than they should (e.g. "Users: Can assign and unassign permissions to users" might give access to "Users: Can create, edit and remove proxies")

• The permissions use the old naming convention. We'd like to update them to be more descriptive and consistent with the names used for more recently developed permissions (e.g. "Users: Can create, edit and remove proxies" will become "Users: User proxy view, edit, create, delete"

Overview documentation of users app permissions: https://docs.google.com/spreadsheets/d/1IJnlWvi4k-wFatDNimx9fQ375evHSIIxtGMookHvs-I/edit#gid=598778404