fix(editor): read vim mode from prefs key and remove toolbar checkbox
The editor was reading api.settings.getKey('vimMode') but the Settings
app stores vim mode under the 'prefs' key as {vimMode: true}. Changed
to read getKey('prefs') and extract .vimMode from the result object.
Removed the vim mode checkbox from the editor toolbar entirely - it
belongs in the Settings app only. Added listener for
editor:settings-changed pubsub to live-reload vim mode when changed
in Settings.