Add ability to migrate all data from one installation to another

[MrCsabaToth]

#169 would allow the migration of one workout, but ultimately it'd be great to migrate everything in one zipped file. This'd include calorie tunes, power tunes, device usages, leaderboard entries. Maybe even settings?

[MrCsabaToth]

Maybe I should just append the Activity CSV together into one file with a well known separator since I already have the code to export one. Then in the import side I could just split first and then invoke the import for each.

[MrCsabaToth]

Regarding settings: Isar will export data in one JSON except the settings. If I'd replace settings with Isar (isar/isar#1066) then I'd need to implement the BasePrefService in https://github.com/DavBfr/flutter_pref/blob/master/pref/lib/src/service/base.dart using Isar if I want to keep using Pref's UI, or use a different UI. This would be a lot of work. I discovered that the said BasePrefService API provides a toMap and fromMap, which we can use to obtain all the settings and serialize it. That's the way to go: we can prepend this to the Isar JSON with some magic separator (like |||) and then upon import dissect it.