shellEditorView: reorganize `renderUpdates` method

section headings should only get rendered if the view is rendering; the
emptyCount and shellCount should be updated if an emptyShell is added at
the end
shellEditorView: bugfix: update all headers

on `renderUpdates`, update all remixerView headers, not just the header
of the first remixerView
shellEditorView: bugfix remixView header prefixes

a header should be prefixed with its index unless it is an emptyShell
placeholder or there is at most one non-empty shell in the collection
shellEditorView: remove redundant method calls

renderRemixerViewHeading gets called by renderUpdates, and renderUpdates
is called when a link changes and after all calls to renderRemixerView
specs: fix test names to describe what is tested

@jbenet be careful that you name tests to describe the functionality
they are actually testing
spec: test remixerView headers in shellEditorView