[SECURITY] Mitigate directly accessible file upload in form framework

File handling implementation in `UploadedFileReferenceConverter` of
`ext:form` creates files in `/fileadmin/user_uploads/` whenever some
Extbase controller is (implicitly) dealing with `FileReference` models,
unless particular implementations assign specific type converters or
register type converters having a higher processing priority.

As a side-effect this could lead to by-passing mime-type validators,
allowing to plant cross-site scripting and other malicious binaries
to public accessible `/fileadmin/` storage. PHP files and similar are
blocked since `fileDenyPattern` rule is active in any case.

This change makes the usage of `UploadedFileReferenceConverter` more
specific in the scope of processing contact forms with `ext:form`

* use random folder names for files, `.../form_abcde12345/image.png`
* removes `UploadedFileReferenceConverter` from being used implicitly
  by other Extbase implementations dealing with `FileReference` models

`PseudoFileReference` has been introduced to limit properties being
serialized to `uid` (in case it's a real file reference) or `uidLocal`
(in case it's a transient reference, pointing to a file).

Direct URLs to uploaded files are substituted by `fileDump` eID script
now, enforcing corresponding FAL mime-type and denying the web server
from guessing/interpreting a different mime-type based on file suffix.

A unique form `__session` value has been introduce, serving as seed
to derive for instance mentioned folder names for uploaded files. In
addition to that, form `__state` is only parsed when having been
submitted via expected `FormFrontendController::performAction`.

Resolves: #92136
Releases: master, 11.1, 10.4, 9.5
Change-Id: I7c33803443a68d6b3c895ec74da802a70bd390c1
Security-Bulletin: TYPO3-CORE-SA-2021-002
Security-References: CVE-2021-21355
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68435


Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>