[BUGFIX] Fix wrong markup of download icon in extension manager

Apply the classes 'btn btn-default' directly to the a tag instead
to have the full button area clickable.

Resolves: #81579
Releases: master, 8.7
Change-Id: Ib296d8e8e56c45d5d879cb7cb7a749b1c7141b2f
Reviewed-on: https://review.typo3.org/53216


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>