Merge to dev: Add more functional tests

[dougbu]

• 180 part 1 of n

• extend first (Dropbox) functional tests
  • rename test class to match sample project; now DropboxCoreReceiverTest
  • add DropboxCoreReceiverTest.WebHookAction_NonGetOrPost_IsNotAllowed(...)
• prepare other (non-Dropbox) samples for functional testing
  • target frameworks specified in $(StandardTestWebsiteTfms) property
  • refactor Programs to meet Mvc.Testing expectations
  • add default secret key to appsettings.json files; add this file if necessary
  • fix BitbucketStronglyTypedCoreReceiver to compile with .NET 4.6.1 and to run validations
• add functional tests of remaining (non-Dropbox) samples
  • does not yet include tests requiring a non-empty request body

nits: VS removed some BOMs

[dougbu]

FYI only. Will merge once CI checks succeed.