Suggestion ignore list

We shouldn't make suggestions for correctly spelled words which aren't in the dictionary.

For example, department acronyms aren't in the dictionary but are commonly used in searches, so let's use the list of organisation acronyms in the search index.

Additionally, there are many acronyms and words which are not in the dictionary but are in common use on GOV.UK, such as "SORN", "apostille", so lets use a text file.