Auto-pin code when uploaded by governance

[ethanfrey]

If it is important enough to have a governance vote to StoreWasm, it is most likely important enough to Pin.

Let's do this by default. Either add a field which must be explicitly set to disable auto-pin (empty is pin), or just always auto-pin and force them to manually vote to unpin if there are too many codes.

Thinking of some concrete use cases here, where this will be a very helpful default.