Bug 1043527 - email UI: Show External Images styling

The "load external images" was moved under the subject now, and since an icon is now part of that section, a flexbox with two children was used so that the icon will stay visible even for longer translations that need multiple lines, and to keep the icon vertically centered with that text.

The existing locale string for one of the display cases for this UI to fix a typo in the English translation: the user cannot "click" as it is a touch UI, so "tap" is used instead.