Fix #351 Make adding channels easier

[KrzysztofMadejski]

Changelog:

• showing channels bar on homepage for admins even if there are no channels
• added + sign (with WCAG message) on a channels bar to link to channel creation
• Channel's language is a mandatory field now
• Channel's and Noun's language is prefilled with current language
• Channel's order field has been hidden and drag-and-drop interface has been introduced by adminsortable2 library
• New translation: Hint for Channel's is_featured has been added
• WCAG "hide element" classes been added to css

[bahattincinic]

thanks - it seems okay in the first glance.

I need 1 week to get this in though.