deposit: display fixes

* Fixes the layout without touching the .container class
  from core Bootstrap and set proper breakpoint

* Adds new styles for the Instructions box

Signed-off-by: Georgios Kokosioulis <georgios.kokosioulis@cern.ch>
We should use here a custom prefix for inspire - see https://github.com/inveniosoftware/invenio/issues/1770
and call it somehow related to deposit form so that the name is recognisable from the global scope. Another thing is that if you don't really need it is better to use classes instead of ids. So maybe ".inspire-deposit-form-container" as. I am not sure here if we use - (dashes) or _ (underscores) in our naming schema. The prefix should be consulted with Javier @jmartinm, but at some point we must start using it.