Migrate Dispatch "Files" to Wagtail "Documents" (after other migrations occur)

[keeganland]

Dispatch "Files" table entries are odd because they don't properly wrap a single file so haven't been necessary to migrate. Still, conceptually those files belong as Wagtail "Documents", which may increase their reusability

[FireBoyAJ24]

@psiemens, at this stage the code base has been sifted and contains no dispatch code. We may have unmigrated and removed all documents from dispatch through #1157.