[TASK] Make the update-from-TER link more visible

Make the "update from TER"-action in the extension manager more
visible by showing a button for it, instead of a text link.

Also moved the rendering functionality of the update link from
a viewhelper to a partial, as partials tend to be more readable
and bug free.

Resolves: #46029
Releases: 6.1
Change-Id: If447bba28bc0dee3e7de286e1a93e062ef59843c
Reviewed-on: https://review.typo3.org/18687
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn