Refactor SpamActionService and its usage

* Removes 'SpamCheckMethods' and makes spam-checking workflow
  explicit in callers
* Moves captcha verification to be encapsulated in 'SpamActionService'
* Introduces 'CaptchaVerificationService', which can be expanded upon
  on the future for additional captcha implementations.
* Moves handling of 'request' parameter to be called only where it
  is needed.
* Removes memoization of GitLab::Recaptcha.load_configurations! -
  it complicates testing/mocking, and it isn't clear why it was
  ever needed - it isn't memoized other places.
* Rewrites issue and snippet service create/update tests to remove
  tests of internal behavior of other services, and instead just
  test their collaborations via mock expectations.
* Expands and improves test coverage of SpamActionService
* Other related changes to accomodate new interface and behavior.