Consider cleaning up $INST/var/tmp on upgrades, to prevent users getting mismatched JSP files

It seems that in some cases the jsp classes in $FISHEYE_INST won't be overwritten on upgrades (potentially because their dates is newer than the date on the jsps in the release). This can cause errors when trying to render the UI.

We should probably delete the compiled JSPs from $FISHEYE_INST when an upgrade occurs, to make sure they are not left over.