[Merge] lp:~postfuturist/mailman/rest-api-mods_owners into lp:mailman

[Stephen A. Goss]

The AlreadySubscribedError I removed is superfluous, as that error will get raised regardless in the call to `subscribe` if that is the case.
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https://code.launchpad.net/~postfuturist/mailman/rest-api-mods_owners/+merge/72977
Your team Mailman Coders is requested to review the proposed merge of lp:~postfuturist/mailman/rest-api-mods_owners into lp:mailman.