models/Issue: fix replace_filter

Update of existing filter of existing issue results in detaching given filter (old version) from other issues it was attached to (it doesn't attach new version). This fix makes sure that old version of filter is detached only from issue being edited. It results in update of filter only for one Issue.

I'm not sure if this is good aproach, maybe I could change logic so filters would be updated for all of the issues, it would be similar as in case of importing and editing filter for new issue. In such case filter is replaced in all of the issues that used this filter.