Use Flask for routing

This commit:
- replaces links that look like buttons with forms and submit buttons
- splits the view code for SMS into its own file
- moves the routing into the Python by adding handling for `post` requests
- uses Flask’s `url_for` to generate URLs, rather than hard coding them (so that
  it’s easier to change the URLs)
- chages the URLs for sending text messages

app/main/__init__.py

@@ -3,4 +3,4 @@ from flask import Blueprint
 main = Blueprint('main', __name__)
 
 
-from app.main.views import index, sign_in, register, two_factor, verify
+from app.main.views import index, sign_in, register, two_factor, verify, sms