Function filterQuery

Normalise and check the filters (normaliseFilter: trimmed, attribute in lower case) and turn them into query parameters (filter[revolvingdoordata][true]=true). Throws LobbyValidationError for a malformed attribute or value, for an attribute outside SEARCH_FILTER_ATTRIBUTES (knownFilterAttributeProblem) — the register would ignore it and return the whole unfiltered set — and for a value outside SEARCH_FILTER_VALUES (knownFilterValueProblem) — the register would match nothing. allowUnknown skips both catalogues, for an attribute or value the register added after this release; LobbyregisterClient.search still verifies that the reply echoes every filter, which also catches an attribute the register has dropped.