Alan Guo Xiang Tan 6a79864f14 PERF: Cache categories query in Stylesheet::Manager.color_scheme_digest.
The query is being executed each time we try and generate the link path
for a stylesheet within the duration of a reqeust. Categories are not
updated that often so repeating this query multiple times a request is
wasteful.

At the time of this commit, there is a `publish_discourse_stylesheet`
ActiveRecord callback on the `Category` model which clears the cache of
`Stylesheet::Manager` each time a category is saved.
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