Allow subclassing InvitationsAdapter

[Flimm]

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Previously, the code would check to see if ACCOUNT_ADAPTER equalled "invitations.models.InvitationsAdapter". But this would cause it to reject subclasses of InvitationsAdapter. Instead, it is enough to check to see if ACCOUNT_ADAPTER setting exists, as this setting is meant for django-allauth.

I would like someone to review this, to check to see if I'm missing something.