Squash repeated auto-saves

When editing the HTML of a text asset, the ``change:body`` event
will be firing much more frequently. Only save when things have
stopped changing.

Addresses #848
Only call renderAssetViews() once on sync

Instead of binding ``SectionAssetView.renderAssetViews()`` to the
``sync`` event of the view's ``assets`` property, only bind it
when needed, prior to calling a ``fetch()``.  This prevents
unneccesary re-rendering of the asset views every time an asset is
saved.

Addresses #880, #847