NEXUS-6045: Move restlet1x under /service/local

PR originally was "Move out static resources from restlet1x. Make their handling not happen in restlet1x, to move away from nexus-restlet1x-plugin".

But, it proved problematic and would not lead us further away from Restlet1x lock-in as it was mounted on webapp root still serving stuff like index page, index redirects, and some static content NOT under /static and finally, the stuff from the WAR itself like favicon etc. So, finally we decided to _really move restlet1x out of our way_.

New static-resources plugin is introduced, that takes over all these tasks from restlet1x, delegates the index page creation to relevant plugin adding "UI specific boot" (currently nexus-ui-extjs3-plugin, but is pluggable). It also serves up the static resources from plugins, WAR and everything else that is "non REST resource" (either restlet1x or siesta). Finally, adds conflict detection to static resources, and improves their reporting too. Now restlet1x plugins handles only the /service/local sub-path of Nexus (and is similar what Siesta plugin does, does not "reach out" of that).

In core introduced error page handling that is used by the content and new static-resources plugin, that is basically "enhanced" from restlet1x plugin (the usual error page Nx was showing). Restlet1x page left as is, but error page feature might be used by other plugins too, that introduces some new servlet, to have uniform error page across whole app.

Issue
https://issues.sonatype.org/browse/NEXUS-6045

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF4