Various art additions.

Placeholder texture for Artur Hawkwing.

Formation modifications for the two Heroes of the Horn currently in the
mod.

Working model and textures for the new Blight feature.
Birgitte, Aes Sedai, and Blight art setup.

Added unit formation and member information for Birgitte Silverbow.

Added an icon and unit definition for Aes Sedai units.

Added Blight feature content and art.
Custom notifications integration.

Pulled in the custom notifications changes to allow for more flexible,
robust notifications.