Summary:
Previously we always created <blockquote class="gmail_quote"> to wrap quoted text. This is not correct.
Gmail uses blockquotes only when it wants visual indentation, and <div>s to wrap other quoted text, like forwarded
messages which are not displayed indented.
This diff updates N1 to match Gmail exactly. Note that for replies, Gmail actually nests a blockquote.gmail_quote
inside a div.gmail_quote.
I also updated signature handling because it turns out the regexp that was removing existing signatures would blow
away any and all divs until it reached a <blockquote> tag.
Test Plan: See updated specs. Manually tested by creating a thread in Google Inbox and then performing fwd and reply in both N1 and Inbox. Results match.
Reviewers: juan, evan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D2750
