Since the principal not necessarily is json-searializable we need to
avoid this and just pass the wanted properties to thymeleaf

fixes #799

closes #792

Using spring.boot.admin.ui.title/spring.boot.admin.ui.brand you can
customise the page title and logo/title shown in the navbar

closes #212

- use MimeMessage with Content-Type text/hmtl
- use a single template for subject and body
- use a Spring aware TemplateResolver

To allow hot reloading for templates and resources add according
properties.
Add instructions in the README.md on how to develop the ui