Factor out string escaping code

So that it only lives in one place.

app/main/views/conversation.py

@@ -10,7 +10,7 @@ from notifications_utils.recipients import format_phone_number_human_readable
 from notifications_utils.template import SMSPreviewTemplate
 from app.main import main
 from app.main.forms import SearchTemplatesForm
-from app.utils import user_has_permissions
+from app.utils import user_has_permissions, unescape_string
 from app import notification_api_client, service_api_client
 from notifications_python_client.errors import HTTPError
 
@@ -133,6 +133,6 @@ def get_sms_thread(service_id, user_number):
 
 def get_sms_content(notification, is_inbound):
     return (
-        bytes(notification['content'], "utf-8").decode('unicode_escape') if is_inbound else
+        unescape_string(notification['content']) if is_inbound else
         notification['template']['content']
     )

app/main/views/dashboard.py

@@ -31,6 +31,7 @@ from app.utils import (
     FAILURE_STATUSES,
     REQUESTED_STATUSES,
     Spreadsheet,
+    unescape_string,
 )
 
 
@@ -204,7 +205,7 @@ def get_inbox_partials(service_id):
             for message in messages_to_show
         }:
             message.update({
-                'content': bytes(message['content'], 'utf-8').decode('unicode_escape')
+                'content': unescape_string(message['content'])
             })
             messages_to_show.append(message)
 

app/utils.py

@@ -379,3 +379,7 @@ def get_cdn_domain():
     domain = parsed_uri.netloc[len(subdomain + '.'):]
 
     return "static-logos.{}".format(domain)
+
+
+def unescape_string(string):
+    return bytes(string, "utf-8").decode('unicode_escape')