Update GUI static file configuration parameters

[MJJoyce]

The HTMLRoot state object currently pulls config from gui.html.directory. This used to be under the section where we customized the GUI. With the current plugin configuration this ends up as a stranded awkward chunk of configuration.

We should move this into the plugin configuration and update this object as needed in the GUI plugin __init__

class HTMLRoot:
    Static = pkg_resources.resource_filename('ait.gui', 'static/')
    User   = ait.config.get('gui.html.directory', Static)