Add custom message type for bastions

[arbron]

Currently the Bastion singleton is responsible for creating the chat messages for bastions and handling events triggered from the bastion chat cards. Adding a new custom chat message type would allow for moving some of this logic out of that monolithic class.

This could either be handled by a single bastion card, or there could be two types to handle the bastion turns and the bastion attacks, since they have different data and rendering requirements.

The turn cards currently store all of their data in flags.dnd5e.bastion as an object containing orders (object with facility ID and order results), items (array of objects with item UUIDs and quantity), and gold (object with gold quantity and claimed boolean).

The attack cards store their data in the same place and include deaths (number of defenders killed) or undefended (no defenders before the attack occurred).