Upgrade comment emails to emit HTML bodies in addition to the existing
plain-text message bodies. The format of the HTML body is remains
similar to the plain-text format, with the following small changes:
* File links are anchor tags rather than plain URLs.
* Parent comment quotations are set in BLOCKQUOTE tags rather than
prefixed by the > character.
* Code quotations are monospaced rather than set in a
proportionate-width font.
* Mult-line code quotations are set in BLOCKQUOTE tags rather than
being prefixed by spaces followed by a colon.
* File groupings are set in nested UL & and LI tags rather than
separating groups with an extra line feed.
Change-Id: I2ac21961b6d48e3fb3455ca6a3e30d86e50b567c
2223cb7c52