[TASK] Move publicly accessible files to typo3temp/assets/

New folder structure is given which contains all files
(images, css, js) which need to be accessible for the
web client.

Also, the folders have been named properly:
- typo3temp/pics/ is now typo3temp/images/
- The typo3temp/temp/ directory which was previously
used for storing temporary images when doing complex
image masking etc. is now named
typo3temp/assets/transient/ which also contains some
volatile files when creating online media records before
FAL processes the file.

Resolves: #72580
Releases: master
Change-Id: I77f49152f09139fc1e42bc1a0ffab101022af43d
Reviewed-on: https://review.typo3.org/45669


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>