Address the missing trailing slash in the `create/instanceId` URL and revisit parsing critical widget/instance information from the window location in creator components.

[FrenjaminBanklin]

The route for editing an existing widget currently does not require (or support) a trailing slash, contrary to every other route the app defines.

Adding a trailing slash to the path definition isn't hard, but this has consequences in the WidgetCreatorPage and WidgetCreator components, where adding an additional slash changes the values parsed from the path.

In the case of creating new widgets this will break the creator completely, since the necessary adjustments to properly handle the trailing slash for existing widgets causes the creator to assume a new widget's instance ID is 'create'.