Organizations: allow uploading avatar

[stsewd]

• The file size is set to 750kb as max
• The image dimensions should be equal or less to 500px
• Only jpg and png formats are allowed (this is a soft check anyway), we are not checking for the content, but django does check if the file is an image using pillow at least.
• Images are publicly accessible and can be enumerated, I think this is fine, avatar images aren't sensible information.
• pillow is a required dependency when using ImageField

This requires creating a new bucket and the corresponding settings in ops.

Closes https://github.com/readthedocs/readthedocs-corporate/issues/1471

[stsewd]

Not sure about the migrations check, I don't see any changes locally :/

[humitos]

Looks good to me. I left some small comments.