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>
1. If I needed to use class instead of id I would have done it but since the markup is already under Invenio template with this id and a class "form-feedback-warning" that doesn't make sense to the purpose we need, it's overkill to create a PR to Invenio for adding a class in order to satisfy our purposes. 
2. A proper solution would be to rewrite the Inspire template which extends the Invenio one with our own markup.
3. Read this http://oli.jp/2011/ids/ and see that the performance of using id over class for 1000 elements is slightly better. 

The lesson is not to sacrifice markup and semantics for CSS selectors. The fix is all about not overriding the core Bootstrap classes that can have a side effect to the whole layout.

Cheers!