R1:00b4eae1f4a8

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…