Merge pull request #716 from pascalchevrel/bug-850431-15-year-anniversary

Bug 854232 - [zh-CN] 15 Years campaign, activate the locale
Refactoring CSS, prefixing properties, animation tweaks

Also updated slides 14 and 15.
Updated sequencejs
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.
Title mark and typography

Also fixed the signup form layout