Greater warning when changing domains with Facebook share buttons

### What

When the domain is changed on a site the number of Facebook shares is lost.

### Why

Where someone has a lot of shares it can be distressing when they see the shares wiped out.

### How

That the numbers are passed over or when changing domain, a warning is included amongst the list of things that will change

Context 5234373-zd

No file, test, or entry point is named. Start by locating the domain-change flow and the Facebook share-button handling, then determine whether share counts can be preserved or whether the domain-change confirmation must warn about their loss. Done means the selected behavior is implemented and verified with an appropriate test.

Written by the indexing model from the issue text.