editSummaryView: bugfix: only save fields on blur

saving (and validating) on keyup caused the field to reset to default
values at awkward times
splashView: access thumbnail through property

bypassing the property to get straight at the data will ignore any
default thumbnails that ought to be used
model: bugfix: use existential operator in getter

Else default values will be returned over falsy, defined values. Also,
switch out `this` for `@` and indent
stylefix: remove trailing semicolon
remixerView: reset link input field on escape
remixerView: cancel link change if field is empty

unless empty fields are permitted
shell datamodel: remove defaultThumbnail property
shells: manage default thumbnail in remixView only

shell models do not need a notion of the default thumbnail, only the
current thumbnail. upon initializing remixView, set thumbnail to default
value if it does not have a value. in shells where the default thumbnail
may change during remixing (e.g. vimeoShell, collectionShell), call
`_updateThumbnailWithDefault`` to refresh the default.
spec: stylefix: correct test block indentation