Attempt to fix <root/> entries that specify a database

[jermdavis]

A first pass at a fix for the issue raised in #97, addressing the ContentMigrator module and a few core bits.

I've tried to rework a collection of API calls and method signatures so that databases are passed about correctly. Which should allow syncing the core database to work. There are probably better (neater) fixes to some of these issues, but the changes in this PR has been done quickly to enable a colleague to get on with some work, rather than be the ideal fix.

It's not exhaustively tested. I've done testing against a "localhost" root - which will successfully expand a core db tree, and preview/sync. (Obvs with no actual changes...) And my colleague who raised the original issue with me has also tested it between separate instances of his solution, and says that appears to be working similarly.

I wasn't able to make the unit tests run correctly in the v1.7 codebase (I get ~50 failed tests with the unchanged code) so I've not put effort into ensuring the tests aren't affected by these changes. The tests compile - but I didn't have time to work out if these failures were because the tests are broken, or because I was doing this in Sitecore v10 and VS2022.

Shout if you need any other work/changes to get this merged.