Move zanata-editor into frontend Maven module

Include zanata-editor in the frontend maven module.

• it can still have a separate package.json and be in a separate subdirectory (i.e. it can be a separate npm module)
• the javascript builds for local development should work sensibly
  • able to build just the npm module that you are working on without having to wait for unrelated parts to build
• mvn package should build all the jar files
  • can be a single jar with all the npm modules under frontend, or separate jars (whichever makes most sense).