REVIEW [NEXUS-5672] Create an ExtJS3 UI plugin

This pull request collects most scattered UI resources into one plugin. Only UiContributor/IndexHtmlCustomizer remains in nexus-core and is changed to honor the `?debug` URL parameter.

A change to plugin-parent will add files from src/main/js to be available from /static/js URL. That's actually against some comments in plugin-parent POM that everything in there should only be active when it's configured in the plugin, but maven-resources-plugin is always used and will always try to copy js files. On the bright side that means we don't need to go update every plugin to include the sources for ?debug mode.

Next step in terms of UI maintenance/improvements would be to add something like https://github.com/pguedes/maven-jsduck to produce js documentation. That plugin is working, but it seems like that particular plugin is not published into a public repository ATM, so I refrained from adding it to the build for now (there's not much documentation in the current sources anyway).