Cockatrice download

[fenhl]

A feature for downloading the files required to play Custom Constructed formats on Cockatrice. Ideally, these files would be automatically generated from the set data and include errata.

[fenhl]

The documentation for custom set files is at https://github.com/Cockatrice/Cockatrice/wiki/Custom-Cards-&-Sets. Not sure how to make images work though.

[fenhl]

Since Cockatrice has support for cards that enter the battlefield tapped, cards matching st:custom o:"~ enters the battlefield tapped" -o:"~ enters the battlefield tapped." require more careful attention.

[fenhl]

The new review process already uses the Cockatrice file generated by the Custom Standard MSE exporter, so generating them is no longer required. Instead, the download should probably just be a link to the Big Custom Box. To avoid losing existing features, support for etb tapped should be added to the exporter.