Cleanups

[petrsvihlik]

A couple things from #212

• refactor return $"{urlPart.scheme}://{urlPart.host}{urlPart.pathBase}{urlPart.path}{urlPart.queryString}";
• review "yyyy-MM-ddTHH:mm:ss.fffZ"
• review ProxyAwareRouteUrl
• replace all occurrences of AccessTokenDefaults.ACCESS_TOKEN_QUERY_NAME
• clean usings
• create tests test/WopiHost.Core.Tests/Security/Authentication/WopiProofValidatorTests.cs
• register the validator in the DI container