Make it possible to configure Elasticsearch index name

Summary:
Similar to storage.default-namespace sometimes during development you'll want
to handle multiple indexes alongside one another. Rather than hardcoding the
/phabricator/ index make this exposed in new search.elastic.index setting,
defaulting to the existing "phabricator"

Test Plan:
Existing installations should be unaffected by this change. Changing the new
setting will result in new indexes being created when someone runs
`./bin/search index` again

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: 20after4, rush898, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9798