Open page dialogue - Search by remote completion when online / local completion when offline

Instead of having a background thread fetching all page names locally, we should implement the following behaviour:

• when the user is online, the "Open page" dialogue completes page names by querying the server (NB: we would need some pause so that it doesn't send a request for each letter you type even if you type them quickly)
• when the user is offline, the dialogue searches within the local page index.