[TASK] Replace clearcache spinner with FA icon

Currently, the spinning wheel displayed after clearing the cache is a
graphic larger than the used font. This caused a "jumping" of the
topbar.

Additionally, adjust the text-align to avoid wobbling of fa-spinner.

Resolves: #62702
Releases: master
Change-Id: Ic17875bb75259aa7615bbe52b43bc912b2b7cf30
Reviewed-on: http://review.typo3.org/33798
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>