2276: Autoconfig of email clients r=mergify[bot] a=nextgens
## What type of PR?
Feature
## What does this PR do?
It provides auto-configuration templates for email clients and encourages them to use implicit TLS (see https://nostarttls.secvuln.info/)
There are numerous caveats:
- it will only work if suitable DNS records are created and certificates obtained (autoconfig, autodiscover, ...)
- the mobileconfig file isn't signed
- the credentials will be prompted... we could/should provision a token on each request instead
- it currently doesn't advertise caldav
- it's IMAP only
### Related issue(s)
- close#224
## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.
- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.
Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>
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