[FEATURE] Introduce Share font to backend for H1

Brings the share web font to backend. Following TYPO3 styleguide
and typo3.org website. Share font is used for module headline only.

Change-Id: Id6f1ff9b7971dff7192928df68677b5a98485cb8
Resolves: #36017
Releases: 6.2
Reviewed-on: https://review.typo3.org/20654
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn