Replace modeladmin use in Incident M2Ms to use SnippetViewSet or ModelViewSet

[SaptakS]

ModelAdmin has been deprecated in Wagtail 5.2 (and will probably be removed from support in next major release), and they are recommending to replace it with SnippetViewSet or ModelViewSet.

There is still https://github.com/wagtail-nest/wagtail-modeladmin that can be used if we decide that we still need it for the Incident M2Ms. ModelAdmin is also being used for CommonTag and EmailSignupModelAdmin