Bug 19860: Make staff client home page responsive

Bug 19860: Make staff client home page responsive

This patch rewrites the markup of the staff client home page to use the
Bootstrap grid instead of the YUI grid, taking advantage of Bootstrap's
built-in responsiveness.

This patch does not affect the logo and search form in the header. It
will be necessary to address search forms in another patch.

To test, apply the patch and clear your browser cache if necessary. For
better results, apply the patch for Bug 19851 as well.

- View the staff client home page and test the layout at various browser
  widths.
- Test with and without news items
- Test with and without content in IntranetmainUserblock

Signed-off-by: Dominic Pichette <dominic.pichette@inlibro.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>