Bug 856922 - Enable Contribute page for Arabic

* Add email address to LOCALE_CONTACTS
* Add locale to LOCALES_WITH_MOZ15
* Add custom banner (css rule+png file)
* CSS rtl:
** fix buttons in nav (font too small, margins), areas of interest (order of elements)
** fix form fields order
** fix checkbox in subscription section
Merge pull request #1600 from flodolo/bug856922_getinvolved_ar

Bug 856922 - Enable Contribute page for Arabic, improve CSS for RTL