[Feature Request] Support base 64 images in dmc.Avatar

[pip-install-python]

Was trying to use dcc.Upload with dmc.Avatar but it wouldn't work, reading through the docs of dcc.Upload I noticed it uses a base 64 image that is capable of working with html.Image, attempted to use this base 64 upload in dmc.Image and it works fine, just would be a nice improvement to have dmc.Avatar also support base 64 images.

[AnnMarieW]

This is caused by an overly restrictive sanitizeUrl.

As with dmc.Image anddmc.BackgroundImage It is not necessary to sanitize the URL of the dmc.Avatar because it renders as an html Img.

See all the details and background discussion in #256