Add SUPPORT_URL definition to os-release
    
    switchboard-plug-about in Pantheon Desktop looks up parts of os-release(5)
    to fill in various values about the operating system. The only value
    we are missing in our os-release file is SUPPORT_URL, which leads to
    broken links in the code. Per FESCo decision, it points to a wiki landing page.
    
    Reference: https://pagure.io/fesco/issue/1699
    
    Signed-off-by: Neal Gompa <ngompa13@gmail.com>