add email code verification

by hitting POST /<user_id>/email-code, we create an email two factor
code to send to the user. That email contains a link with a token that
will sign the user in when opened.

Also some other things:

"email verification" (aka when you first create an account) doesn't
hit the API anymore

refactor 2fa code verification and sending to use jsonschema, and share code between sms and email

Die marshmallow die!

app/user/users_schema.py

@@ -0,0 +1,35 @@
+post_verify_code_schema = {
+    '$schema': 'http://json-schema.org/draft-04/schema#',
+    'description': 'POST schema for verifying a 2fa code',
+    'type': 'object',
+    'properties': {
+        'code': {'type': 'string'},
+        'code_type': {'type': 'string'},
+    },
+    'required': ['code', 'code_type']
+}
+
+
+post_send_user_email_code_schema = {
+    '$schema': 'http://json-schema.org/draft-04/schema#',
+    'description': 'POST schema for generating a 2fa email',
+    'type': 'object',
+    'properties': {
+        # doesn't need 'to' as we'll just grab user.email_address
+        'next': {'type': ['string', 'null']},
+    },
+    'required': [],
+    'additionalProperties': []
+}
+
+
+post_send_user_sms_code_schema = {
+    '$schema': 'http://json-schema.org/draft-04/schema#',
+    'description': 'POST schema for generating a 2fa email',
+    'type': 'object',
+    'properties': {
+        'to': {'type': ['string', 'null']},
+    },
+    'required': [],
+    'additionalProperties': []
+}