Move make_query_from_params to XapianQueries

This is involved with the construction of meaningful xapian queries with
respect to InfoRequestEvents. This commit also removes the
get_tags_from_params method, which presumably was targeted at
PublicBodies, but doesn't seem to actually be used anywhere.
XapianQueries is used to extend InfoRequestEvent in order to prevent
InfoRequestEvent becoming too unwieldy and to preserve the association
between these methods.
Cache frequently accessed request lists.

Make specs a bit more focused, remove view specs - they're not relevant to the new code in their current form and don't seem to merit updating.