Add loading indicator

So that a user knows something is happening display a loading indicator
while the Ajax request is performed.

Add `aria-busy` to let screen readers know something is happening.
Improve narrow tablet screen styles

- Removing the A-Z title for narrow tablet sized screens mean long words
  won't be longer than the column they have to be displayed in.
- Increasing the minimum height for narrow screens means the subsection
  border and background will always overlap the whole of the root list
Add IE specific stylesheets

This lets them get desktop styles rather than mobile stylesheets. Also
add in IE specific varables for JavaScript incase they are needed in the
future.

Add the new stylesheets to the precompile so they are on production.
Merge pull request #27 from alphagov/ie-fixes

Add IE specific stylesheets
Merge pull request #25 from alphagov/tablet-fixes

Improve narrow tablet screen styles
Merge pull request #23 from alphagov/loading-indicator

Add loading indicator
Update breadcrumbs on navigation

When you enter a subsection add the section to the breadcrumb trail.
When you leave the subsecion remove the breadcrumb.