Switch to PostgreSQL in development, to match prod
Fix a bug that could crash the admin page

Certain numbers were coming out nil on an empty database; this just
protects against nil.
Fix performance problem on ActiveAdmin user search

Search results page was including a dropdown that included every
individual Location, thousands of them, which took ~15 seconds to render
locally. Switched this to a text search.
Add spoof button to ActiveAdmin user page

Part of the ongoing migration from old, custom admin stuff to
ActiveAdmin. This already exists on the old admin user page; just adding
it to the ActiveAdmin one.
Only show link to request detail when appropriate

On a fulfillment item, only show the link to the request detail page if
the user is actually the fulfiller. In particular, don't show it for
unfulfilled requests (where the fulfillment is actually a new, unsaved
record), because the volunteer won't be able to see the request until
the actually become the fulfiller.
Add link to request detail from fulfillment detail

Also tweak copy for consistency
Add student location to fulfillment detail

This is sometimes useful if the student neglects to include their city
as part of their mailing address.
Add warning/reminder to sign in as donations@