[TASK] Move backend templates to extension contexts

Most html templates in backend are used in one extension only.
Those templates should be held in extension context to reduce
dependencies.

Creates extbase/flow directory structures.

Change-Id: I7638092b695d36c4cea2a2755d8285bf92114bb0
Fixes: #47786
Releases: 6.2
Reviewed-on: https://review.typo3.org/20374
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn