added definition to the top of concept pages

seems like crucial information

of course configurability makes this much harder than it first appears
(cf. inline comment); TBD
moved concept sidebar to the bottom

caveats:
* the "create concept" link is now gone - but it never seemed right
  there anyway, so we need to find it a new home
* eventually, these links should be displayed horizontally
* RDF representations should have dedicated icons:
  http://lab.linkeddata.deri.ie/2011/rdf-logos/
added RDF icons

as permitted by http://www.w3.org/RDF/icons/

caveats:
* should probably be a background image, but I had some trouble sizing
  that properly
* not sure which Sass module this belongs into
introduced view model to move logic out of template

there's a risk that we're just introducing yet more indirections (like
iQvoc needs more of those... ), but it does seem more manageable by
providing a more immedate overview of available properties - however, it
remains to be seen how this approach fares with more complex branching,
as that _might_ lead to awkward duplication and dependencies

for the moment, I opted not to use a separate library (e.g.
https://github.com/drapergem/draper) as this seems straightforward
enough to not warrant such additional complexity

caveats:
* for some reason `auto_link` doesn't work (cf. inline comment)
* definition class and index are hard-coded (cf. inline comment)
* context (`ctx`) passing should not be necessary, but I wanted to avoid
  `instance_eval` - not sure there's an elegant solution here
* only a few properties have been ported to `@view` so far - eventually
  we probably want _everything_ to go through @view
* that `if...else` for representation icons belongs might seem like it
  belongs into the view model instead - but that would introduce HTML
  into that layer, which seems wrong

/cc @mrreynolds