Add job ad: Web Designer

Part of https://github.com/cargomedia/mgmt/issues/19

Job ad to find a frontend developer who can implement our designs (from Selim or external designers) in html and css. They should be able to do that in fashion so it's easy to maintain, reuse and extend. They should learn to understand the structure of how our views compose a page (smarty templates, isolated css).

We currently have an ad "Front-End Developer" which can be replaced then?

What should be the title? "Web Designer"? "Front-End Developer"? What are others using?

@christopheschwyzer could you look into this so we can start looking for someone as soon as possible? Greg and John are available to help you with contents and English language.