Fix reply email parsing for linebreaks in "On <date>, <user> wrote:" quote identifier

Summary:
Move the parser to a separate class so it can be easily unit tested, add some
tests. Properly parse emails with linebreaks in the quote line.

Test Plan:
Ran unit tests, used mail receiver to reply to an object.

Reviewed By: cadamo
Reviewers: aran, jungejason, tuomaspelkonen, cadamo
CC: aran, cadamo, epriestley
Differential Revision: 392