Bug 980461 - [MMS] Visual adjustments to attachment download indicator.

WIP patch, wait for bug https://bugzilla.mozilla.org/show_bug.cgi?id=963013 to be landed. Some concerns:

* Date format in message is different, if patch for bug 963013 isn't going to fix that, we need separate bug for it;
* Do we need separate hover\active styles for download button\link?
* What font is used in visual spec? Looks like it's a bit different than we use in code;
* Spinner should be updated by patch for bug 963013.