Rough cut of inline comment context

Summary:
Ref T10694. This is still missing some pieces, but seems to get most of the data into the mail in a plausible format:

• When an inline remarks on code, show the patch inline in the mail body.
• When an inline replies to another inline, show that other inline in the mail body.
• Apply remarkup rendering to inline content.
• Apply basic styling to mail body blocks.

Not covered yet:

• Syntax highlighting.
• Diff highlighting.
• Maybe clearer style/layout hints to connect comments to what they reply to? Current approach might get messy with inlines that have blockquotes and code blocks inside them, for example.
• I probably want to cap the amount of diff context we ever show to ~7 lines, even if you drag over 200 lines of code.
• CSS is a generally a bit rough still.
• The unified-comment-context option is effectively always on now, and should be removed.
• Text section is getting indented right now but probably shouldn't be.
• Spacing, etc., might be a bit off.

Test Plan:
Rigged Home to render these things, got a plausible-looking render (top is text, bottom is HTML):

{F1259052}

Sent myself some inline comment mail, got a plausible result.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10694

Differential Revision: https://secure.phabricator.com/D15850