[FEATURE] Rework email notification for workspaces

Sending out emails when items have been processed on
a stage change is now done via Fluid Email, allowing
administrators to fully customize both subject
and body of an email.

All emails sent by default contain more information and
better structured contents instead of marker-based templates.

The following TSconfig options apply to customize the emails:

- tx_workspaces.emails.stageChangeNotification.generatePreviewLink = 0
- tx_workspaces.emails.layoutRootPaths
- tx_workspaces.emails.partialRootPaths
- tx_workspaces.emails.templateRootPaths
- tx_workspaces.emails.format = html (or "text" or "both")

Resolves: #90411
Releases: master
Change-Id: I837c1538230ed55f2e34d51288e3b943fdd65238
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63248


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>