Index attachment content

https://www.pivotaltracker.com/story/show/44917843
https://www.pivotaltracker.com/story/show/43217457

Indexing the attachment content with Tika, which is what ES attachment mapper plugin uses, so that we 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.

https://github.com/alphagov/rummager/pull/134 (merged)
And a rummager reindex 

https://github.com/alphagov/whitehall/compare/master...index-attachment-content