Disallow "{", "}" and "#" in Project monograms

Summary: Currently, mentioning two projects in a block (`{#a} {#b}`) produces an overzealous parse. Forbid these characters in project monograms.

Test Plan: Got correct markup.

Reviewers: btrahan, dctrwatson, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D8141