Additional request state: Pre-Approval

Many travel policies, for example, require that certain expenses be
approved before tickets can be purchased. An example from Conservancy's
travel policy include: hotel bookings beyond the GSA/Dept-of-State Per
Diem hotel rate, and flights that exceed the with-$100-of-cheapest rule.

As such, requestors need the ability to request preapproval.

These changes herein committed, however, do *not* account for the fact
that a request may already be "In Progress" when another expense comes
up. An example of that is a flight was booked already in policy and the
requestor, and uploaded, and the requestor then discovers later that the
hotel is out-of-policy and needs preapproval. We can perhaps ignore
this scenario for the first specification of this to avoid
feature-creep, but I wanted to flag it as a potential issue for future.

The work around might be that the Bookkeeper is allowed to move a
request between any state to another, so the work-around in this
specific instance may have to require an out-of-band conversation
between bookkeeper and requestor. That's not disaster.