REVIEW [NEXUS-5672] Create an ExtJS3 UI plugin

https://issues.sonatype.org/browse/NEXUS-5672

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. Plugin javascript will then be loaded in the single (dev) files instead of the minified version. By using XmlPluginRepository + symlinking resources/static/js to target/classes/static/js you can edit the js in place and only need to reload the Nexus UI, no need to restart Nexus for UI dev anymore.

This pull changes setup for yuicompressor plugin, so every plugin contributing UI will need to move their js/css files to src/main/resources/static/(js|css) and needs a dependency on the new UI plugin to pick up UiContributor/IndexHtmlCustomizer.

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).