Add the ability to collapse multiple sections at once

0. Testing this requires hacking.

1. Edit theme/bootstrapbase/layout/columns3.php, adding this code @ different places, say 3 times:

• one @ before the end of the </header> tag.
• another after outputting the "side-post" blocks.
• the last one before the end of the </body> tag.
  Code to add:

  <div class="nav-collapse collapse">This should be collapsed in narrow window</div>

2. With the patch applied, caches purged and using any bootstrap-based theme, go to the site main page.

3. With full-width (>1000 aprox), you will see the 3 occurrences of the code above.

4. Narrow the window till the collapse-toogle button (up-right) is shown. The 3 occurrences are not visible now.

5. Press the collapse-toogle button, the 3 occurrences are shown in their original places.

(without the patch applied none of the occurrences is shown ever when the window is narrowed, aka, point 5 fails).