XD-1375 Create first implementation of UI design docs

* Theme the Spring XD UI to look like Spring.io
  - Use LESS and Bootstrap variables (as much as possible) to keep styles maintainable
* Provide very rudimentary login support (solely for supporting the UI)
* Provide responsive support
* Add fonts
  - Not using Google fonts via CDN as XD may be deployed without net-access
* Add quick filter to Definitions tab
* Add new background image to start page
* Upgrade Bootstrap to version `3.1.1`

Jira: https://jira.spring.io/browse/XD-1375