#187 Populate avatar from SAML attribute

[Lumrenion]

This PR addresses https://github.com/nextcloud/user_saml/issues/187.

• In admin configuration, a new configuration field avatar_mapping is added.
• If avatar_mapping configuration field is not empty, the user wont be able to upload an image in the nextcloud profile settings
• The avatar_mapping field holds a publicly available URL to the profile image of the user. As usual, the image will be downloaded to nextcloud when the user logs in via user_saml.

[AkarinServer]

What time can this PR be merged into main branch?

[solracsf]

@Lumrenion conflicts and DCO need to be solved. Thanks 👍

[solracsf]

/rebase