Bug 819557 - Enable Get involved page for Turkish (tr)

Usual stuff:
* add localized banner + css rule
* add email address and locale code to 15 years template

Both pages are still not activated on stage/prod, since I'd like to
enable them once this PR is available on production (if I activate them
now the page will show up with the old template).
Improve download help screenshots for Bug 918913
 - Update screenshots to reflect the stub installer on Windows (Bug 918913)
 - Use the new logo in the windows screenshots (Bug 921264)
 - Adjust the size of the screenshots to fit on the grid
 - Sprite the download screenshots and use JPG (faster/better)
 - Use wider/shorter aspect ration to leave more room for text below
 - Update responsive styles accordingly
Merge pull request #1311 from sgarrity/bug-918913-new-download

Improve download help screenshots for Bug 918913
Merge pull request #1352 from flodolo/bug819557_tr_getinvolved

Bug 819557 - Enable Get involved page for Turkish (tr)