Use default avatar for User/Orgs in OAuth services

[humitos]

Example:

Closes #3353

[jlumbroso]

@agjohnson @humitos @ericholscher I am wondering what is the current state of avatars on readthedocs?

For instance, as a maintainer of the comma project, my avatar is the default blank:

I have connected my GitHub account to my RTD.io account, and it contains an avatar. I have also looked through all the settings dialogs to see whether I could upload my own avatar. On the other hand, I have noticed many others have an avatar (this project for instance), and I am not sure what they are doing differently—or this is a bug.

[stsewd]

@jlumbroso this github avatar is only shown in the import list, for the avatar itself we use the one from gravatar (probably we should document this)

[jlumbroso]

@stsewd Many many thanks for the super speedy response! That's exactly what I needed to know!

In terms of documentation, I would suggest simply adding a mention in the "Profile" editing section, saying

Avatars are sourced from Gravatar using your Primary Verified email address. To configure your avatar register an account on Gravatar with the proper email address.