Extract attachment text using tika

Using tika to extract the text from attachements, as it supports more
file types than just PDF.

This is preferable to using the attachments-mapper plugin as it avoids
storing the actual attachment binary in the search index.

We might be able to ignore the attachment title, as it's highly likly
to the be first thing in the document
only return the output if the system call was sucessful

There is an edge case if the file has not been virus scanned yet.

Also testing on some files show processing errors, so only add to index
if sucessfull text extraction has occured
Test for attachment data text extraction

Refactor code into attachment data
Move attachment title into the attachments rather than indexable_content

All fields are being searched, so we can move the titles from indexable
content into the attachment data

see rummager:
https://github.com/alphagov/rummager/blob/master/lib/elasticsearch/searc
h_query_builder.rb#L138