[BUGFIX] Restore enable/disable functionality in beuser compare

This patch restores the possibility to enable backend users in
the compare view again. It also reinstalls the check for online
users to prevent disabling a currently logged in user.

Resolves: #90488
Relates: #90298
Releases: master
Change-Id: I0919403a41de136c7461e0d9367b3795bbf68358
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63371


Tested-by: Susanne Moog <look@susi.dev>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>