Make navigation.js load at the top

And ensure that mobile.head module loads at the top too. This might fix
bug 53511 or at least make it less annoying.

Bug: 53511
Change-Id: I81945dd0723b4c6fb67cc5e8e687a5aa3000ad2e
Introduce header-loaded event

Use it to bind the left nav opening JS as soon as possible. In future
can be also used for binding search earlier.

Change-Id: I0955a418068123634d5d067bc3ddbcfccb33291a
Bug: 53511
Merge changes I0955a418,I81945dd0

* changes:
  Introduce header-loaded event
  Make navigation.js load at the top