Make Maniphest project prefill more modern and standard

Summary: Fixes T4777. We technically support `?projects=...` already, but parse it in an unusual way and apply old, awkward, excessively strict lookups to it.

Test Plan: Used reasonable, standard, human-readable strings to prefill `?projects=` and got the results I expected.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4777

Differential Revision: https://secure.phabricator.com/D8733