[BUGFIX] Use correct content types in backend ajax and eID responses

Due to the recent changes in the commit:
    e487cf52
    [TASK] Create own response instance in controller actions

..ajax routes and eID handlers that used a *pre-generated* Response
object (from the RequestHandler) now return different Content-Type
headers than before.

For backend ajax request applicaton/json was set by default,
for eID scripts no Content-Type was set (by default).

Change these controllers to use JsonResponse or a plain Response
to reflect the previous state..

The changes in this commit were intended to be squashed into the
mentioned commit – but this commit was too late. Therefore other (a bit)
unrelated optimizations to changes that patch made are included.

Change-Id: Icfdcd02d353dfaf48ad959c50be4802349eaaacd
Releases: master
Resolves: #83946
Related: #83939
Reviewed-on: https://review.typo3.org/55766


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>