Make account mappings easier

[egabancho]

For the account info mappings make a default function to do a simple one to one mapping, more complicated things a function should be provided.

Something like:

SSO_SAML_IDPS = {
    "idp-abc": {
        "mappings": {
            "origin_field": "invenio_field",
        },
    },
    ...
}

(Extracted from #3)

[mb-wali]

closed with #18