Make documentation link in Central push warning dialog clickable

[lognaturel]

Software versions

master (post #872)

Problem description

#872 introduced a new warning dialog with a documentation reference when pushing a form with multiple versions to Central. Currently the documentation reference is just plain text. Ideally it would be clickable.

Steps to reproduce the problem

1. Pull a form with submissions from multiple different versions. To simulate this, you could put versioned.zip in the forms folder of your Briefcase directory
2. Configure a Central server
3. Initiate a push of the versioned form
4. See the confirmation dialog

Expected behavior

Clickable link

https://github.com/getodk/briefcase/pull/872/files#diff-81f1259745a11e2b9ff53e749c1322de33b8738f808d457ad78b6d42c85d0fafR101 is where the dialog is launched.