[FEATURE] Form: "Redirect" postprocessor

* Implements a basic postprocessor to perform a redirect.
* Adds it to the form-wizard.
* Enables prepared ExtJS-code for removing a postprocessor.

Resolves: #44482
Resolves: #38094
Releases: 6.2
Change-Id: I7e09bc9aa9821ac20e890343eb95c34c010dfba3
Reviewed-on: https://review.typo3.org/18307
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn