Previously, in a war deployment, the web environment’s property sources
were initialized using the servlet context after the application’s
configuration had been read by ConfigFileApplicationListener. This
meant that spring.config.location configured via the servlet context
had no effect.

This commit adds a new ApplicationListener,
ServletContextApplicationListener, that initialises the configurable
web environment’s property sources using the servlet context. It’s
ordered with higher precedence than ConfigFileApplicationListener to
ensure that any properties defined in the servlet context are available
when loading the application’s configuration.

Closes gh-6801