Standardize response codes and add tests to SMS keys

[sethvargo]

This cleans up a few nits from the introduction of sms signing keys.

• Make "features" a top-level struct available in all templates instead of passing in specific features
• Update some language and HTML on actual sms keys page
• Rename page and nav to "Authenticated SMS"
• Check user-error vs server-error and return appropriate response codes
• Add more tests, including tests for database failures

Release Note

Standardize response codes and add tests to SMS keys

/assign @mikehelmick
/assign @whaught

[google-oss-robot]

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[sethvargo]

/retest