uSync: import previews and 17.5.2-prerelease.15

Repo: KevinJump/uSync

v17: content and media imports get a visual preview in the details dialog, next to the changes list (#1168, into #1166)
v17: details dialog tabs renamed to Changes / Visual
v17: details dialog has previous / next buttons to step through every change in a report
v17: content preview info follows the import settings
Pre-release 17.5.2-prerelease.15 on the nightly feed, and the first npm pre-release as @jumoo/usync (next-17) via trusted publishing
Merged #1169: SignalR respects the skip client negotiation setting (thanks timoschraa)
Merged v17 forward into v18 and v19