Move inline import to top of file

Usually we have imports at the top. It looks like the reason for
them being inline was to avoid a circular import, but we can also
avoid this by not importing everything from the app module.

Since we're about to add more imports from event_handlers, now is
a good time to refactor them. Note this matches how we import the
event handlers in every other module.

app/event_handlers.py

@@ -1,6 +1,6 @@
 from flask import request
 
-from app import events_api_client
+from app.notify_client.events_api_client import events_api_client
 
 EVENT_SCHEMAS = {
     "sucessful_login": {"user_id"},