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.

Needs this PR: https://github.com/alphagov/rummager/pull/134
And a rummager reindex 

https://github.com/alphagov/whitehall/compare/master...index-attachment-content