Gracefully handle the Elasticsearch server being unavailable

[Princess Celestia]

In a community set up to use Elasticsearch, the Elasticsearch server becoming unavailable for any reason, even temporarily, causes Invision Community to begin throwing unhandled exceptions. It's most obvious on user profiles which turn into public-facing internal server errors when this happens.

This situation would make sense to handle more gracefully by detecting it and, at minimum, doing at least the following:

1. Show an alert to administrators that the Elasticsearch server is inaccessible, similar to the ones that appear when the Redis server or background tasks aren't working correctly.
2. Gracefully degrade user profiles and other areas of the suite that depend on the search engine to display everything they can which doesn't require the search engine (profiles without activity feeds or with empty activity feeds, for example).