Bug 1028224: email UI: settings chevrons and Load More as a button

These fixes are for these 2.1 bugs:

* Bug 1043529 - email UI: "Load more messages" styling
* Bug 1043533 - email UI: account settings styling

Since they were trivial HTML/CSS changes, I grouped them together to avoid extra process/review burden.

For the chevrons in the settings menu (the > to the right of the items that have another card associated with them), I looked for a /shared style for it, but there was none, and I just copied what Settings does: just create a special style for it referencing the /shared image for it. Since it is in the "buttons" section of shared styles, and we already include that .css (so that the build will also pick up the images for that "buttons" section), we will have the image in our app bundles.

See [bug 1028224 comment 10](https://bugzilla.mozilla.org/show_bug.cgi?id=1028224#c10) for the UX specs for these two items, and here is an image that shows the resulting visual display for the fixes:

![chevrons-and-load-more-button](https://cloud.githubusercontent.com/assets/73359/3724851/5f56353c-1682-11e4-8431-96fd39791b42.png)