Make two small fixes to the quoting feature

Summary:
Ref T4119.

  - Trim newlines off the quoted text before quoting it; otherwise we can end up with a staircase of ">" at the end of a quote.
  - Allow image macros to have leading whitespace, so multiple consecuitive quoted macros work properly.

Test Plan: I QUOTED MACROS A LOT OF TIMES

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4119

Differential Revision: https://secure.phabricator.com/D8983