Resolves fdo#81581: Mediawiki doesn't recognize underlining

Cherry-picked from b8f4db5bad245c2e340b3217b10df994785eecd3

Change-Id: I27ec27bd733e5161bbd18ba7cf813daa3ac6c089
Reviewed-on: https://gerrit.libreoffice.org/10437
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>