Accept a captcha score threshold for challenges from the spam filter

...rg/whispersystems/textsecuregcm/WhisperServerService.java

...ystems/textsecuregcm/controllers/ChallengeController.java

...stems/textsecuregcm/limits/RateLimitChallengeManager.java

...hispersystems/textsecuregcm/spam/PushChallengeConfig.java

...stems/textsecuregcm/spam/PushChallengeConfigProvider.java

...ms/textsecuregcm/controllers/ChallengeControllerTest.java

...s/textsecuregcm/limits/RateLimitChallengeManagerTest.java