Rolling chain / snapshot import possibility for Sia-UI

[scenox]

As the size of the blockchain grows, it's harder for new users to jump into the eco system.
Not everyone has the patience to wait days until Sia-UI is ready to be used.

Some possible solutions:

1. Rolling chain - instead of downloading the whole chain, just download the last x blocks (as option)
2. Provide easy way to download and use secure snapshot of the consensus db.