Relaxed <requireJavaVersion> configuration of the maven-enforcer-plugin

This commit resolves #695 by changing the configuration so that all Java versions from 1.7 and above are allowed.
Ensure path starts with "/" in ErrorController

When mapping the ErrorController path to Spring Security it's
important that it starts with "/". This change ensures that is
the case even if the user has omitted the leading "/".

Fixes gh-694
Add fixedlocaleresolver based on application property

If the user sets spring.mvc.locale and doesn't provide a @Bean
of type LocaleResolver then a FixedLocaleResolver will be provided.

Fixes gh-697, fixes gh-669