[Feature/Improvement] Import/Export Workouts

[mondlicht-und-sterne]

An option to export or import workouts. Also from others. In addition, not everyone has to enter everything.

[trymeouteh]

I would like to see a way to create JSON files which can be opened from a folder on your device and allowing you to save new files on the device.

This way, a user can backup the files as they wish and using other tools like Syncthing, the user can sync their JSON files to other devices such as a tablet and not need to import and export their workout files.

[a-mabe]

Closed in #153