[FEATURE] Enhance confirm/accept actions

To give valid feedback to users that e.g. clicked confirm/accept
links more than once, the frontend user should be submitted as
well. This way, users e.g. get feedback, that their user account
already has been created.

All confirm/accept templates have been extended by using "user"
as arguments for the accordant links. The RealURL configuration
has been updated as well.

Resolves: #60165
Change-Id: I1b8a88fe6204eb4582199bfd441ffa0f1c692d89
Reviewed-on: https://review.typo3.org/31458
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer