XD-1616 UI: Add username/password screen

* Add Thymeleaf template
  - Configure `Gruntfile.js` to build `login.html` and copy to Thymeleaf template directory during Grunt build
* Add Spring MVC controller to handle login
* Polish login related styles in `main.less`
* Convert `spring-xd-ui` module to Java module (Spring Boot)
  - Allows for pluggable UI (`spring-xd-dirt` simply declares a dependency to the `spring-xd-ui` module)
  - Clean-up `spring-xd-ui` related tasks in `build.gradle`
  - Allows the UI (and the Thymeleaf templates) to run in isolation
  - Remove admin ui controller configuration from `/src/main/java/org/springframework/xd/dirt/rest/RestConfiguration.java`

Jira: https://jira.spring.io/browse/XD-1616
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