Add 'simple' option to fx download button helper. Bug 1053637.

- Implement simple button on /firefox/new/.
- Add placeholder high res android image (just a copy of standard
  res renamed).
- Compress both android images on /firefox/new/.
Merge pull request #2236 from jpetto/bug-1053637-simplify-dl-button-fx-new

Add 'simple' option to fx download button helper. Bug 1053637.