[CLEANUP] Topbar indicator

This patch fixes some graphical glitches with the loading
indicator in the topbar:

 - Old spinner icon overlays the fa-pendant in some cases
 - fa-spinner wobbles due to different measurements

Releases: master
Resolves: #63091
Change-Id: I8c8afb1ebed4518157dd89234da17c9573213aeb
Reviewed-on: http://review.typo3.org/34376
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>