If the build version of the instance is present in the metadata it's now
also considered and the version is now a attribute on the instance
itself. So when using non-boot application they can provide it via the
registration metadata.

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

The WebClient's responses should be always consumed, so that underlying
buffers are released.

Fixes #666

this ensures that the application name gets escaped

closes #313

unecessary scss sections removed
no type for errors props

The previous code snippet contained code which would go in the `distributionManagement` section of a POM, meant for where to publish snapshots. It is however intended for users to use in their POMs to access published snapshot versions of the Spring Boot Admin artifacts. This change makes that more clear and gives the correct snippet to copy-and-paste into one's POM.

This commit enables client and server to work out of the box on
CloudFoundry.
The client derives the registration info (url and metadata) from the
VCAP environment variables.
The server computes a unique instanceId and the correct headers for the
cf router based on the metadata provided by instances. Also suppots the
CloudFoundry discovery client ootb.

closes #434

When running on any CloudPlatform the auto-deregistration defaults to
true otherwise the behaviour is unchanged.

closes #658