Merge #883

883: Admin create user enhancement r=mergify[bot] a=cr1st1p

## What type of PR?
Enhancement

## What does this PR do?
It allows the admin docker image to also create the admin user.
The idea is that in my kubernetes setup, I do not want to manually do anything, as such, I need a way for the admin user to also be created automatically without me getting inside the pod.
So I had to change the manage.py function that creates the user to allow different 'modes' (me, I'll be using 'ifmissing') and also start.py to call that functionality if appropriate environment variables are present.

So now, in my Deployment, I add 3 more environment variables and I get the admin user created, IF not already present.

### Related issue(s)
none?

## Prerequistes
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: place entry in the [changelog](CHANGELOG.md), under the latest un-released version.


Co-authored-by: cristi <cristi.posoiu@gmail.com>
Co-authored-by: cr1st1p <cristi.posoiu@gmail.com>
Co-authored-by: Tim Möhlmann <muhlemmer@gmail.com>

CHANGELOG.md

@@ -13,6 +13,7 @@ v1.6.1 - unreleased
 - Enhancement: Make Unbound drop privileges after binding to port
 - Enhancement: Stop using static assets, but build them using Webpack
 - Enhancement: Create an Authentication Token with IPv6 address restriction ([#829](https://github.com/Mailu/Mailu/issues/829))
+- Enhancement: Automatically create admin user on container startup if given appropriate environment variables
 - Enhancement: Missing wildcard option in alias flask command ([#869](https://github.com/Mailu/Mailu/issues/869))
 - Bug: Fix creating new fetched accounts
 - Bug: Fix poor performance if ANTIVIRUS is configured to none.