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>
deposit: support of multiple subjects

* Allows to select multiple subjects using the Bootstrap
  multiselect library for the frontend.

NOTE: run `grunt` to copy the external libraries.

Signed-off-by: Georgios Kokosioulis <georgios.kokosioulis@cern.ch>