migrate to webjars for frontend resource management

- petclinic.css is now auto generated, therefore it is put in the
ignore list
- vendor folder is removed in favour of webjars
- remove several maven dependencies which are already managed by spring
boot
- unfortunately, lesscss maven plugin does NOT support importing from
classpath, as a result, bootstrap is unpacked during process-resources
phase, we need need to find a better resolution
- remove bower integration