Do not sync the avatar picture if the user manually set it in XWiki

[trrenty]

The application currently syncs the azure avatar with xwiki everytime the user logs in, overriding any possible modification.

If a user coming from azure changed their profile picture, it would be nice for the application to take that into consideration and not override it.

[ChiuchiuSorin]

To be implemented in a similar way as LDAP authenticator: if a user changes the avatar locally, it will not be updated from the provider.