requests: fixing comments notifications for member requests

[slint]

Update: these were implemented, merged, and released, but then reverted, since they are backwards incompatible. Options for next steps:

• Address via Explore a template inheritance solution for having a "base" template invenio-notifications#32
• Address via making a major release on the affected packages

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Request comments on member invitations and membership requests are not working because the recipients can be communities, that the base comment notification builder cannot resolve to specific user email recipients.

The default set of community recipients should be managers and owners (since they're the that have permissions to accept/reject member-related requests).

(Note, see how a similar fix was applied at #1256)