Support loading the UI from a directory

The UI loader should support loading the UI from a directory. This can be especially useful for debugging and rapid prototyping. It may also make it easier to integrate the UI directly into the playbook project for users who prefer that configuration.

The UI loader should assume that the directory mirrors the structure of a UI bundle.

Done criteria:

• The UI loader detects that the UI bundle option is a directory (only if the URL check fails)
• The UI loader loads the bundle from the specified directory
• The UI loader throws an error if the specified directory does not exist