Support for dark mode

This update fixes an issue where if Fastmail folders are not found (for example, on your calendar page) no count is shown instead of 0.

Fastmail changed their HTML structure, which caused an error for mailboxes that were empty. Now it works again!

Update the icon to Fastmail's latest design, and make it work with high-dpi ("retina") displays. This update also uses whatever icon Fastmail provides, so it should update by itself next time Fastmail changes their design.

Version bump to update screenshots and description on the add-on page.