New filter_id query

[mdashlw]

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Introduces new search query filter_id:123 that will retrieve and match the specified filter. filter_id inside filters is not allowed.

filter_id:1 - shows images not hidden by filter 1
NOT filter_id:1 - shows images hidden by filter 1
filter_id:1 AND filter_id:2 - shows images that pass both filters (i.e. not hidden by either)
filter_id:1 OR filter_id:2 - shows images that pass either one of the filters (i.e. not hidden by both)

Also: moderator fields are now allowed in watchlists

Should there also be filter_name that searches system and user filters?

[liamwhite]

if there's only one point where the pattern diverges, case is usually preferred over with

query_string
|> parse_filter_ids()
|> load_filters(context)
|> case do
  {:ok, filters} ->
     {:ok, Map.put(context, :filters, filters)}

  err ->
     err
end

[mdashlw]

I did it like this in case prepare_context ever needs to do more work than load filters but I can change it if you want