[TASK] EXT: Belog -  backend module H1-H4 (html)

Restructures headlines in EXT: belog backend module to
reflect hierarchy of the content: H1 through H4.

Change-Id: I3d462bbbbc7a79d13da154dca9e393a204e9ae29
Resolves: #48539
Releases: 6.2
Reviewed-on: https://review.typo3.org/21049
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters