Sends only one notification per batch review

What does this MR do?

Instead of receiving an email for every note in the batch review, a user should only receive a single email with all the notes bundled in it.

Changelog entry added, if necessary
Documentation created/updated
Tests added for this feature/bug
Conforms to the code review guidelines
Conforms to the merge request performance guidelines
Conforms to the style guides
Conforms to the database guides
Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
EE specific content should be in the top level /ee folder
For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

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