Add address completion to gnus layer

[jhenahan]

The mu4e layer has very pleasant address tab completion. I'd love to use Gnus because I like the way it handles lists and so on, but I haven't been able to get a decent working configuration going for good address completion. I've tried bbdb, but I haven't been able to get it to do anything automatic based on the documentation I've found around the internet (mainly involving insinuate 'gnus and the like). Adding every one of my contacts manually sounds like a nightmare, and the vCard import option leaves everything formatted very oddly.

If someone knows something useful I can add to my config so that I can harvest addresses from my local dovecot instance or something, that would be fine, too, but if there's something we can roll into the layer, that'd be even better for future users.

[github-actions]

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