[BUGFIX] Decouple thumbnail generation from controllers

Deferred rendering of thumbnails via an external request speed up
the backend performance. This currently works in all places that use
BackendUtility::thumbsCode.

Resolves: #85605
Releases: master, 8.7
Change-Id: Id5635fc9cd340b8bcf21995cd457415aaa1a8926
Reviewed-on: https://review.typo3.org/56765


Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>