[WIP] Add "City Editors" and "Mutli-City Editors" staff groups.

This adds two new groups that staff can be classified as:

* **City Editors** are staff who have the ability to edit all kinds of information about the city their organization is affiliated with. All users, organizations, organization member types, and individual roles associated with their city can be added and edited. City Editors don't have the ability to delete most of these objects, though (although they can mark many as inactive).

* **Multi-City Editors** are like City Editors, but can edit the information for any city.

Things still left to do:

- [x] Make it possible for these editors to change flatpages relevant to their city/cities.

Wrinkles introduced by these changes:

* City editors can "vouch" for users by affiliating them with an organization; however, once this happens, editors for other cities can't see that user, which could result in confusion if, say, a user has accidentally been mis-associated with the wrong organization in the wrong city (this could also happen via the auto-vouching system too). This means such city editors would have to find a Multi-City Editor to fix things for them, which is annoying.