Add a method to convert portals when their style is changed

[potatolain]

Users are able to change portals to use different block types at any time by changing the configuration file. Unfortunately however, existing portals may stop working at this point, since they rely on the existing blocks. A new command should be created which reforms all existing portals using the new structure; replacing all existing blocks around known portals.

Suggested command: /portal convert

Alternatively, we could do this automatically somehow.