Rename SMS to send

Because this view is no longer just for sending SMS messages, it and its
associated tests should be renamed, so `send_sms.py` becomes `send.py`, etc.

app/main/__init__.py

@@ -3,7 +3,7 @@ from flask import Blueprint
 main = Blueprint('main', __name__)
 
 from app.main.views import (
-    index, sign_in, sign_out, register, two_factor, verify, sms, add_service,
+    index, sign_in, sign_out, register, two_factor, verify, send, add_service,
     code_not_received, jobs, dashboard, templates, service_settings, forgot_password,
     new_password, styleguide, user_profile, choose_service, api_keys, manage_users
 )