Add in attachments on the government index

Using Tika to preprocess the attachments you can store an array of the
attachment text to be included in the search index.

We are not using the attachments-mapper plugin because this stores the
base64 encoded version on the file (twice) and IG has 40GB of files

Extend the attachment model to include extra fields to index
Merge pull request #134 from alphagov/index-attachments

Add in attachments on the government index
Revert "Merge pull request #134 from alphagov/index-attachments"

This reverts commit 27a0660c61003513c82d3864ae1e63074fe2b6a8, reversing
changes made to b1c6588b26c14cd7dbbe6a114a6a3fb150b76cb5.