Constraints on spatiotemporal extents in wizards

[jdries]

The wizards in the web editor currently nudge users towards creating fairly large spatiotemporal extents, which is a common source of failing jobs.
Our general constraint in openEO platform is that we test up to 100x100km at 10m resolution, but this is not super easy to check client side for arbitrary collections.

So here's two suggestions:

• run /validate by default at the end of the wizard, to show potential warnings/errors generated by the backend
• make default extents more on the safe side, for instance by zooming in a lot more on the map, and settings last month as temporal extent?