Factor out the code that parses metatag values (e.g. `score:>5`) and
search URL params (e.g. `search[score]=>5`) into a RangeParser class.
Also fix a bug where, if the `search[order]=custom` param was used
without a `search[id]` param, an exception would be raised. Fix another
bug where if an invalid `search[id]` was provided, then the custom order
would be ignored and the search would be returned with the default order
instead. Now if you use `search[order]=custom` without a valid
`search[id]` param, the search will return no results.
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