Cross-reference "Create a deployable war" how-to

Add a tip for both the Maven and Gradle "Packaging executable jar and
war files" section referring to the how-to.

Fixes gh-2086
Set the default session timeout to 30 mins

Update AbstractConfigurableEmbeddedServletContainer to set the default
session timeout to 30 minutes. Also correct Javadoc to specify that
the default is '30 minutes'.

Fixes gh-2084