Support relaxed names with ConditionalOnProperty

Update ConditionalOnProperty to optionally support relaxed form names.

Fixes gh-835
Add spring.resources.add-mappings property

Add `spring.resources.add-mappings` allowing users to configure if
resource mappings should be added or not.

Fixes gh-804
Extract WebMvcProperties

Extract WebMvcProperties from WebMvcAutoConfiguration and also
update conditionals to use @ConditionalOnProperty.