Use Twilio Verify API for phone attestations

[tomlinton]

Checklist:

• [x] Code contains relevant tests for the problem you are solving
• [x] Code has been formatted with autopep8 --in-place --recursive --a --a .
• [x] Ensure all new and existing tests pass
• [x] Update any relevant READMEs and docs
• [x] Submit to the develop branch instead of master

Description:

• Replaced the phone attestation with the Twilio Verify API.
• Replaced some PhoneVerificationErrors with ValidationErrors where appropriate, so that field names can be passed along and feedback displayed to the user in origin-dapp.
• Cleaned up the unused phone column from the verification_code table.
• Cleaned up unused twilio package.
• Add responses package to mock responses from Twilio for testing.

This resolves Issue #95 (also includes further discussion of this change). It depends on PR #272 from origin-js. Also PR #110 will ensure ValidationErrors result in the right HTTP responses this was merged into develop.

The settings of the development deployment of origin-bridge will need to be updated to accomodate this. The TWILIO_VERIFY_API_KEY replaces the other Twilio settings. The API key can be generated here. The application name is important because it determines what the text to speech engine says when it calls you. I used OriginID which worked fine.

[micahalcorn]

Thanks, @tomlinton 😬 It looks like there are a couple of significant merge conflicts that should be solved.