register_new_matrix_user: Support multiple config files

[V02460]

Support passing multiple config files to register_new_matrix_user. Before, all but the last config file were silently ignored.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
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• Code style is correct (run the linters)

[anoadragon453]

Thanks for tackling this, and sorry for the slow feedback on review.

This looks good to me, other than one small thing below!

[anoadragon453]

Apologies for being so slow to get back to this. Requesting review is definitely the best way to get a PR back on to my radar. But occasional pings help too (and are welcome!).

One small remaining point below.

[anoadragon453]

This LGTM. Thanks!