When PHPExcel is not installed, detect it and provide install instructions

Summary:
Depends on D18957. Ref T13049. To do Excel exports, PHPExcel needs to be installed on the system somewhere.

This library is enormous (1K files, ~100K SLOC), which is why we don't just include it in externals/. This install process is a little weird and we could improve it, but users don't seem to have too much difficulty with it. This shouldn't be worse than the existing workflow in Maniphest, and I tried to make it at least slightly more clear.

Test Plan: Uninstalled PHPExcel, got it marked "Unavailable" and got reasonably-helpful-ish guidance on how to get it to work. Reinstalled, exported, got a sheet.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18958