Switch to inlining styles.

Unfortunately Gmail for iOS does not respect the <style> block that desktop
Gmail does. Styles are instead defined in a styles.json and can be referenced
via a custom template function. Not quite as nice as CSS (e.g. link style
invocation has to be repeated for every link), but still tolerable.
Fix bottom corners of the commit rounded box getting cut off.
Do not panic if we have a timezone that we can't load.