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)
Issue 1307: tabzilla "innovations" displaying weird
Fix bug 931287 - Make lightbeam page localizable
[bug 926020] correct date on steering committee list
Merge pull request #1348 from nukeador/l10n-ready

bug 931287 - Make lightbeam page localizable
Merge pull request #1361 from alexgibson/bug-926020-correct-date-on-steering-committee-list

[bug 926020] correct date on steering committee list
[fix bug 932371] Update copy on Lightbeam site
Merge pull request #1358 from kyoshino/issue-1307-tabzilla-font

Bug 933310 - Tabzilla "innovations" displaying weird
Merge pull request #1362 from alexgibson/bug-932371-update-copy-on-lightbeam-site

[fix bug 932371] Update copy on Lightbeam site
Bug 933925 - Lightbeam: use the same lang file for all pages (templates)
Merge pull request #1363 from flodolo/bug933802_lightbeam

Bug 933925 - Lightbeam: use the same lang file for all pages (templates)