closes #697

The system.load.average.1m metric is not available on all platforms.
Therefore we ignore errors from fetching it.

closes #697

Fixes stalls in threaddump view due to not completing responses - only
reactive applications were affected so far.

The TaskScheduler used for periodically registering is removed from the
application context and moved to the ApplicationRegistrationListener.

see #478

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.