Offer migration from Briar GTK
Reading the reactions on the fediverse, there seem to be quite some users on Briar GTK currently. With #23 we decided to switch the database's location, resulting in Briar GTK users having to start from scratch.
We should offer a dialog on the first start of Briar Desktop, asking if users want to re-use their data from Briar GTK (if we detect an instance). In case they want, Briar Desktop would copy everything from the old to the new location.