Use single tile mode for mini-maps on search page.

**DO NOT MERGE** (need to address notes below)

The effect of this is that only a single request is made per map (which is ok, we don't need buffering because the mini-map cannot be panned).

Notes:
* doesn't work when hitting tilecache.emii.org.au (not sure why)
* we have 'HiRes_aus-group' only in tilecache (not in the geoserver)
* layer URL still hard-coded

If we can resolve the above, I think this would be a good fix to take a bit of load off our infrastructure.

Comments, @danfruehauf @julian1 @pmbohm @pblain ?