Metadata files should contain relative file paths

[ggalmazor]

Problem description

Current code in master generates metadata.json files with absolute paths to the form files, which make them not portable. Any path should be relative to the briefcase directory.

Steps to reproduce the problem

1. Pull a form and close Briefcase
2. Move the SD to another location
3. Open Briefcase and change the storage location directory in the settings tab.
4. Try to export the form

You should see that Briefcase freezes and there's an error in the logs

Expected behavior

Briefcase shouldn't freeze or fail.