webhooks/github: Improve logic for page build messages.

Some of the page build message code had insufficient test coverage.
I looked at generating the payloads that would allow me to test
the lines of code in question, but it was too much work to
generate the payloads and this seemed like a vague event anyway.

So I just rewrote the logic so that the lines missing
coverage are implicitly covered.

zerver/webhooks/github/view.py

@@ -241,17 +241,19 @@ def get_release_body(payload: Dict[str, Any]) -> str:
 
 def get_page_build_body(payload: Dict[str, Any]) -> str:
     build = payload['build']
-    action = build['status']
-    if action == 'null':
-        action = u'has yet to be built'
-    elif action == 'building':
-        action = u'is being building'
-    elif action == 'errored':
-        action = u'is errored{}'.format(
-            CONTENT_MESSAGE_TEMPLATE.format(message=build['error']['message'])
-        )
-    else:
-        action = u'is {}'.format(action)
+    status = build['status']
+    actions = {
+        'null': 'has yet to be built',
+        'building': 'is being built',
+        'errored': 'has failed{}',
+        'built': 'has finished building',
+    }
+
+    action = actions.get(status, 'is {}'.format(status))
+    action.format(
+        CONTENT_MESSAGE_TEMPLATE.format(message=build['error']['message'])
+    )
+
     return u"Github Pages build, trigerred by {}, {}".format(
         payload['build']['pusher']['login'],
         action