Report: restyle the dropdown menus.

Use the bootstrap-select work of Silvio Moreto, work under MIT license.
See <http://silviomoreto.github.io/bootstrap-select/>.
Report: add multiple select on contract filter.

It rocks!
Report: add multiple select on source filter.

Also change the displayed text into the dropdowns when more than one
item is selected. Before: "CDD, CDI, Internshit". Now: "3 of 4 \
selected".