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/templates/views/check-sms.html

@@ -17,7 +17,10 @@
 			{{ sms_message(message_template) }}
 
 	  	<p>
-		  	<a class="button" href="/jobs/job" role="button">Send {{recipients|length}} text messages</a>
+				<form method="POST" enctype="multipart/form-data">
+					<input type="hidden" name="csrf_token" value="{{ csrf_token() }}" />
+			  	<input type="submit" class="button" value="Send {{recipients|length}} text messages" />
+				</form>
 	  	</p>
 		</div>
 	</div>