uSync 17.6.2, v19 forward-port

Repo: KevinJump/uSync

Released uSync 17.6.2 to NuGet, with the v17.6.2 GitHub release
17.6.2: dependency lookups don't throw on a non-container key (#1202), content previews read the incoming template from the file (#1203), file groups labelled in reports (#1204), failed imports named after the item, not its file (#1205)
v19: merged v18/main forward (#1212); nightly versions now based on the branch's own major (#1213)
Handler sets can read and write their own folders (#756), PR open (#1211)