Use Spring OAuth2 Resource Server over custom implementation
As part of the 5.2.0 release of Spring Security, we can use the
OpaqueTokenIntrospector feature of Spring Security OAuth2 Resource
Server to replace a number of security-related features we had
previously created with out-of-the-box Spring ones.
This requires an upgrade to spring-security and spring-boot-starter (as it pulls in underlying Spring Web changes) to use the new functionality.
Closes #53 (closed).