Added a check to integrate with the flatpak package

[LyzardKing]

This PR adds the flatpak support in the native messaging script used by the web extension.
It has been tested locally and will need a change in the flatpak to auto-install the json file with the correct path (like the other packages do).

I'll update the changelog and the documentation.

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

Thanks for the work!

[LyzardKing]

To add the native-messaging script to the flatpak the easier solutions seem to be:

• Running a script that copies the files to the ~/.mozilla directory before launching jabref
• Adding an option in the settings that copies the files.
  The files can only be added to the user's directory, not the system wide one.

In the first case the script would run a mkdir to check that the directory exists and then a cp to copy the file (if it does not exist).