uSync 17.6.0, 17.6.1 and 18.3.0 released

Repo: KevinJump/uSync

Released uSync 17.6.0 to NuGet, with the v17.6.0 GitHub release
Released uSync 18.3.0 to NuGet, with the v18.3.0 GitHub release
Both: the change details dialog previews items with the real backoffice editors, current and incoming side by side with the differences highlighted. Covers data types, document/media/member types, content and media, via a dry run of the serializer (#1166, #1168)
Both: previewing an item whose doctype isn't on the site yet returns a failed preview instead of a 500 (#1189)
Both: @jumoo/usync published to npm, 18.3.0 as latest, 17.6.0 as latest-17
v18: CHANGELOG.md gets 18.3.0 and the missing 18.2.1 entry (#1191)
Released uSync 17.6.1 to NuGet, with the v17.6.1 GitHub release
17.6.1: item previews use the same handler set as the import (#1193), callers can choose where previews come from (#1192) and set the side headlines (#1194), and the source shows on the left with side colours (#1195)
v18: merged v17/main forward (#1190)