Cleanup params in back link when not using history.back()

[lemeryfertitta]

Discussed in #65 (comment)

When using the "back to filter selection" link in the case where history.back is not used (e.g. a shared search result) we populate the filter page URL with all of the current params. Most of these params are invalid for that URL, so we should likely just trim this to contain only the relevant params.

This is only a small cosmetic cleanup, so probably not a super high priority, but should be an easy fix.