[BUGFIX] Show backend login spinner when getting RSA pubkey

[BUGFIX] Show backend login spinner when getting RSA pubkey

The spinner displayed in the login button of the backend module is now
shown right after triggering the first AJAX call. Previously, it was
shown after the RSA public key was fetched.

Resolves: #71291
Releases: master
Change-Id: I24b74e3812ea69bc74f1372e902785352a90822b
Reviewed-on: https://review.typo3.org/45261
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Rudy Gnodde <rgn@windinternet.nl>
Tested-by: Rudy Gnodde <rgn@windinternet.nl>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>