Index attachment content

https://www.pivotaltracker.com/story/show/44917843

Indexing the attachment content with Tika, which is what ES attachment mapper plugin uses, so that we do do not have to send a base64 encoded version of the file to ES, which then gets stored (and cannot be configured to not store the file), this way we only store the plain text.

Currently only on editions, but we could add this to other searchable models.

Needs this PR: https://github.com/alphagov/rummager/pull/134