support only the selection of one user

closes #12693

Change-Id: I4597471b1691d380f8eeb743e3fd1d621859ffd3
Merge branch 'review/kay_winkler/2132' into 2132_merge

Conflicts:
	linotpd/src/linotp/i18n/de/LC_MESSAGES/linotp.po

Change-Id: Idcbca0eb2eb61a705c694608e9cdec0fbdddf2d3