Merge branch '472988-rte-bug' into 'master'

Fix issue in RTE related to adding text before a mention

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159685



Merged-by: Martin Wortschack <mwortschack@gitlab.com>
Approved-by: Thomas Randolph <trandolph@gitlab.com>
Approved-by: Tiger Watson <twatson@gitlab.com>
Co-authored-by: Himanshu Kapoor <hkapoor@gitlab.com>