Add mobile auth redirect to custom URI scheme (zulip://).

This makes it possible for the Zulip mobile apps to use the normal web
authentication/Oauth flows, so that they can support GitHub, Google,
and other authentication methods we support on the backend, without
needing to write significant custom mobile-app-side code for each
authentication backend.

This PR only provides support for Google auth; a bit more refactoring
would be needed to support this for the GitHub/Social backends.

Modified by tabbott to use the mobile_auth_otp library to protect the
API key.

tools/lib/capitalization.py

@@ -25,6 +25,7 @@ IGNORED_PHRASES = [
     r"Kerberos",
     r"Mac",
     r"MiB",
+    r"OTP",
     r"Pivotal",
     r'REMOTE_USER',
     r'Slack',