Various bug fixes and improvements

- Bug fix: allow access to form sub URLs for permitted roles
- Bug fix: always allow access to captchas
- Bug fix: fix user manager UI
- Feature: add error messages to REST API responses
- Feature: display error messages in forms
- Improvement: harmonize default js scope (limit to molgenis namespace)
- Improvement: harmonize document ready method signature (use "$(function() {})")
- Improvement: Use context URL in js instead of hardcoded plugin URLs
- Improvement: Add etags to static resources