Bug 950208 - [meta] Email Visual Refresh

The visual refresh for email. Most of it is just CSS changes, some things to note:

**compose.html**: the HTML/CSS for the addresses changed for two reasons:

* while adding more addresses, the address area should scroll, but keep the address to/bcc/cc label and the add button visible.
* those sections also had different CSS style, with a darker background. Flexbox was used to get the desired layout for the address sections.

**message_list.js**: There was overlap of the text with the star or attachment icon showing, so if those are shown, the sizes of the text boxes are changed via addition of a css style. I preferred this than using something like flexbox and disabling the display of the icon divs, as that would lead to more size adjustments, and I want to be sensitive to scroll performance since those message items are in the vscroll list.

message_reader and compose both show different icons based on attachment types, with a default icon used for anything not having a special icon. The mime type is used to make that call.

Some of the CSS selectors used are more specific to avoid the use of (discouraged) !important in the CSS rules.

All the icons should be nice an sharp for a GAIA_DEV_PIXELS_PER_PX=1.5 builds for the flame. GAIA_DEV_PIXELS_PER_PX=2 should also look nice and sharp.