2591: Add button to mailu-admin in roundcube task menu r=mergify[bot] a=ghostwheel42
## What type of PR?
feature
## What does this PR do?
Adds a button to the roundcube interface. This button gets you back to the admin interface.
### Related issue(s)
- Replaces #2367
Co-authored-by: Alexander Graf <ghostwheel42@users.noreply.github.com>