refactor(webapps/build): use grunt to build client side assets

This commit refactors the build process for camunda webapp.
From now on we use grunt to build all client side assets, manage
client side dependencies and fetch them.

Grunt is integrated into the build process via the frontend-maven-plugin.
It plugs into three phases

- in the generate-resources phase it builds client side assets and publishes
  them into the directory target/webapp
- in the test phase it tests client side assets (currently stubbed)
- in the package phase the maven-war-plugin grabs the front-end assets from
  target/webapp instead of src/main/webapp

Closes CAM-1712