Web app: Add setting page for data serializer of PythonJob

[superstar54]

If you have multiple plugins that register the same data serializer, the workgraph will raise an error. User can avoid this by selecting the plugin that you want to use in the configuration file.

{
    "serializers": {
        "ase.atoms.Atoms": "abc.ase.atoms.Atoms"
    },
}

Save the configuration file as workgraph.json in the aiida configuration directory (by default, ~/.aiida directory).

Add one setting page to view and edit this configure file.