[Scout] Add alternative titles to search index

[pushrbx]

Related to #189 and #227

jikan/#397 mentions the need for alternative titles in responses, which I think also implies that people would want to search for documents with them too.

[irfan-dahir]

To update/note here (reference #227)

We'll start by indexing only Title, Title English, and Title Japanese.

We can include other languages in the search such as synonyms, german, french, etc (available on MAL) post-merge of #227 and further tweaking/testing.

[pushrbx]

This one is still missing. We already have the data in the database in the titles attribute, which should be indexed by flattening all the titles in a string array.