[l10n] Bug 796062: Get Involved page for Tamil (ta)

* Add Tamil (ta) to LOCALES_WITH_MOZ15.
* Add Tamil's contact to LOCALE_CONTACTS, minor fixes (proper
alphabetical order, fixed alignment on first item).
* Add localized banner to media and css.
* Add CSS fixes for slides 2, 10, 14, 15 (text displayed out of
background)
Merge pull request #961 from flodolo/bug796062-tamil-15years