remixerView: render subview through helper method
remixerView: centralize remixView initialization
remixerView: stylefix: initialization submethods are private
remixerView: stylefix: fix spacing, simplify names
styling: remove double-thick borders in btn-groups
remixerView: add clear button
tests: viewOptions paradigm in shellEditorViewSpec
shellEditorView: add methods to manage degenerates

`_shellIsStub` tests whether a shell is both the final shell and a
defaultShell. `_lastNonDefaultShellIndex` returns the index of the last
shell that is not a defaultShell.
shellEditor: more flexible empty shell management

accommodate empty shells in the data model by stripping empty shells
from only the end of the collection when returning the shell data and by
ensuring there is always an empty shell stub at the end of the
collection
remixerView: don't disable EmptyShell toolbar
remixerViews: only hide stub remixer's summaryView
tests: remove accidental console.log