Bug 967785 - Update several Firefox Accounts images. r=ferjm,arcturus,arthurcc

  * Update image in FxA System app 'enter email' landing screen.
  * Update image in FTE FxA panel.
  * Add image to FxA Settings panel logged-out state.
  * Also fix bug 967783: add final FxA envelope image to Settings, System.
  * Remove flat fox styles from System app, to be replaced with a TBD icon
    in bug 967779.
Merge pull request #18608 from 6a68/bug-967785-update-fxa-landing-image

Bug 967785 update fxa landing image