1015: Merge DNS hints, and hostname/domain ordering hints. r=Nebukadneza a=toxik-io
## Enhancement - Hints in setup, references in setup & DNS docs
Unrelated: This PR makes changes to the Flask setup utilities to suggest ordering the `HOSTNAMES`
entry so that the first entry is the admin panel's domain (at least in my experience that's what fixed my
issues)
It also makes changes to the documentation, referencing that more than the usual DNS
entries need to be set (and the admin panel used) if they've enabled DMARC/SPF/DKIM.
### Related issue(s)
- My original issue #1002
## Prerequistes
- [x] In case of feature or enhancement: documentation updated accordingly
- [ ] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.
## Assistance
I'm unsure if the hints for HOSTNAME ordering are simply because of how the config is gulped in
by the current tooling, or if it's a bug that the frontend only accepts connections from the first host...
I also don't know what commands in the CLI would replicate the steps taken on the admin dashboard.
Further, I don't know what you'd put in the changelog.
Co-authored-by: toxik-io <thetoxicarcade@gmail.com>
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