Add a synonym of Brexit for EU exit
Add a synonym of Brexit for EU exit

This ensures that searches for brexit return results that have the word
EU Exit in the searchable fields.

On production there are far fewer results for brexit than there are
for EU Exit:

17234 results for [eu exit](https://www.gov.uk/search.json?q=eu%20exit)
1337 results for [brexit](https://www.gov.uk/search.json?q=brexit)

This should surface much more content when searching across gov.uk
for brexit, even if brexit has been omitted from the documents
searchable fields.

A schema migration will be necessary after this change across all
environments:
See https://github.com/alphagov/rummager#changing-the-schemareindexing.

alphagov/rummager,alphagov/rummager