Bug 878448 - Use new mail address for German contribute form

Update contribute mail address for German form on Get Involved page.
Fix bug 653876: Add l10n helper for images.
Merge pull request #907 from pmclanahan/add-image-l10n-helper-bug-653876
[Fix Bug 879867] Return 403 on csrf failures
Merge pull request #951 from Sancus/master
Merge pull request #936 from flodolo/bug-878448-new-mail-de

Bug 878448 - Use new mail address for German contribute form
Bug 724682 Redirect MATHML