Add a `words` column to the tags table. This will be used for parsing
tags into words for word-based matching in autocomplete.
For example, "very_long_hair" can be parsed into ["very", "long", "hair"].
The `array_to_tsvector(words)` index is for performing wildcard
searches. It lets us do e.g
SELECT * FROM tags WHERE array_to_tsvector(words) @@ 'hand:* & hold:*'
to find tags containing the words "hand*" and "hold*".
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