[WIP] Remove empty query params from issue sort / filter.

Before this PR, clicking on `sort` at the issues index would give a URL with many empty parameters like:

    assignee_id=&label_name=&milestone_id=&scope=all&sort=newest&state=opened

Now, `assignee_id=&label_name=&milestone_id=` only show if set.

`scope` and `state` should also be removed, but implementation was more delicate here so I might leave this to another PR.