Import sheets from file

[kuroidoruido]

This issue cannot be done before the issue #11 will be merged or at least done.

The goal here is to have an import button in the My Sheets tab of the sheet selector page. The user will be able to select a .vrpg.json then add character sheets from the imported file to existing ones. If the character is already present, we need to rename the character to include the import date (or maybe add to character sheet model the last save date and display it when multiple characters have the same name for the same sheet).