Fix bug 850431 15th Anniversary slide show on /contribute

commit from pascalc: use 15years.lang as an extra lang file for the campaign. Split a sentence in two l10n blocks because that's how I gave it to localizers

commit from jpetto: Added stub for GA tracking slide views.

Will track 3 separate actions - auto, click, keydown.

commit from jpetto: Added GA tracking code.

commit from pascalc: Bug 854232 - [zh-CN] 15 Years campaign, activate the locale

commit from craigcook: Fix validation errors in contribute form

"true" isn't a valid value for the required attribute (it's a boolean attribute)

rows and cols attributes require a positive integer value; the empty string isn't valid.