Updates to georeferencing interface

[mradamcox]

A few changes in this PR

• After some internal discussion and input from @mhpob in Allowing 2-GCP georeferencing: how should we do this? #403, we've decided to make the Helmert transformation with 2 GCPs the default for everyone. No need to manually switch to that transformation or have an extra level of permissions. Personally I was worried about making it too easy to make poor fits with 2 gcps, but I've changed my thinking on that. Requiring 3 GCPs does definitely lead to wonky results on very small pieces. (I did retain the confirmation modal that recommends adding a 3rd if possible when you submit only two).

Other small improvements:

• If a layer has already been georeferenced, zoom to its full extent, not the extent of the GCPs
• If a layer has already been georeferenced, don't show the key map by default