[CLEANUP] Streamline EXT:about

Streamlined EXT:about extension as a first step to clean up the whole
TYPO3 Core base:

- Moved locallang file to EXT:about, as it is only used in this extension
- Removed usage of TYPO3_LOADED_EXT/ext_emconf.php in favor of PackageManager
(needed to add the authors into each Package)
- Fully added all external libraries in use (not including -dev requirements)
- Moved EqualHeight.js to EXT:about as it is only used in this extension
- Proper usage of Fluid layouts
- Streamlined code, variables, imports and comments
- Reviewed comments in PHP

Resolves: #79948
Releases: master
Change-Id: Ia6c2d6b519bc34847131f06908c2c441b9248a8b
Reviewed-on: https://review.typo3.org/51790


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>