[BUG] Creature model doesn't inherit from `HasDescription`

[calumbell]

Bug description

Currently the Creature model doesn't inherit from the HasDescription abstract model. This causes two issues:

1. It is not possible to include creature descriptions, where they are included as openly licensed content (see [DATA] Add missing Monster descriptions to SRD 5.1 Miscellaneous Creatures #934)

2. The HasDescription class also includes the model fields for handling cross-references, which we definitely want on Creatures for linking to spells from stat-blocks

[calumbell]

Closed by #949