Bug 1037635 - Implementation of new mark as read/unread icons in email app

Changes were:

* clean up the getElementsByClassName work so each button is bound to a member variable first, then the addEventListener is done on that member variable.
* remove msg-btn-active toggling since that class it not used anywhere in CSS. I believe it was a holdover from a previous style of marking non-button elements that were active for use, instead of just setting a .disabled on a button. Now button .disabled toggling is used.
* Make sure to disable buttons if no messages have been been selected.
* Toggle the unread/read button icon according to selection. See [bug 1037635 comment 5](https://bugzilla.mozilla.org/show_bug.cgi?id=1037635#c5) for the UX-approved state logic and picture of the state. That picture's set of captures can be used instead of flashing this change on device since those captures where taken from a device.

All the images were run through png_compress.sh before committing them.