This commit replaces Spring Boot's basic dependency management support
with separate dependency management plugin. This has a number of
benefits including:
1. A Maven bom can be used rather than a custom properties file
2. Dependency management is applied transitively rather than only to
direct dependencies
3. Exclusions are applied as they would be in Maven
4. Gradle-generated poms are automatically configured with the
appropriate dependency management
Closes gh-2133
Spring IO Platform already provides dependency management for JavaMail.
This commit updates Boot’s new JavaMail dependency management to align
with the Platform, thereby allowing the Platform to inherit Boot’s
dependency management instead of defining its own.
This commit adds a new starter to auto-configure a MailSender when the
necessary classes are present and when the property "spring.mail.host" is
set.
The auto-configuration also accepts any arbitrary properties that
JavaMail might need using the "spring.mail.properties" prefix.
Fixes gh-1760
