When PHPExcel is not installed, detect it and provide install instructions
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.
Reviewed By: amckinley
Maniphest Tasks: T13049
Differential Revision: https://secure.phabricator.com/D18958