[BUGFIX] Make Install Tool forms submitable via keyboard

Additionally fix missing input-label association and simplify
Fluid inline if usage.

Resolves: #76363
Releases: master, 7.6
Change-Id: I8a01047c3eb5b28494313fce0e031a296696ece9
Reviewed-on: https://review.typo3.org/48391


Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>