Use a single footer block for all pages
    
    The footer situation is a bit of a mess. There is no footer
    in layout.html. There is a footer block with a 'last updated'
    note in base_nav.html, a footer block with no 'last updated'
    note in index.html, and then various pages inherit from
    base_nav.html but override the footer block to be empty.
    
    I think this is partly because the 'last updated' stuff in the
    base_nav footer isn't always working / relevant, but it's quite
    easy to handle that. This commit puts a single footer block
    which works correctly for all cases into layout.html and cleans
    up all the other templates to just use it. This means we get a
    consistent footer across all pages, with the 'last updated' info
    included when it's available and not included when it isn't. It
    also makes the templates cleaner overall.
    
    I changed the text color to white because 'muted' is almost
    impossible to read on my screen. Oddly, in Bodhi, 'text-muted'
    is much more readable, but I think Bodhi is on an old version of
    fedora-bootstrap. Pagure uses white text in the footer so I
    think we can too!
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>