[BUGFIX] Prevent installing incompatible extensions without confirmation

Currently it is possible to install incompatible extensions without
confirming the overlay popup. This patch prevents submitting the
overlay if the button is still disabled.

Resolves: #80005
Releases: master, 7.6
Change-Id: Ia68539cb9ce2a82f1e90aef3823f60cfadee2ce1
Reviewed-on: https://review.typo3.org/51834


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jasmina Ließmann <code@frauliessmann.de>
Tested-by: Jasmina Ließmann <code@frauliessmann.de>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>