[TASK] EXT: be_user - improve listing

Backend user's username and real name are tightly coupled. These
texts should stand right next to each other. Merged username and
real name columns. When real name is unknown, not a problem.

Also adds link to the primary action / edit view to user listing.

Change-Id: I16d06771ad302fe64232e068e12663d87c07d79d
Resolves: #50999
Resolves: #50994
Related: #50998
Releases: 6.2
Reviewed-on: https://review.typo3.org/23020
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn