Add fusermount scripts

[darkdragon-001]

See issue for discussion about implementation.

TODO add documentation to set device=all manually if this functionality is desired as I don't plan to make the hole in the sandbox even bigger by default.

Closes #27

[flathubbot]

Started test build 64572

[flathubbot]

Build 64572 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/47206/io.kopia.KopiaUI.flatpakref

[flathubbot]

Started test build 64580

[flathubbot]

Build 64580 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/47214/io.kopia.KopiaUI.flatpakref

[flathubbot]

Started test build 64588

[flathubbot]

Build 64588 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/47223/io.kopia.KopiaUI.flatpakref