Allow users to choose a profile image

[Changaco]

• add an avatar_src column to the participants table with a custom enumeration type ('auto', 'custom', 'gravatar', 'bitbucket', 'facebook', 'github', 'google', 'twitter')
• wire up the user interface
• if the user chooses custom: check that the file is an image and isn't too big, store it somewhere (relevant: Team images gratipay/gratipay.com#3750)

[Changaco]

I don't want Liberapay to host avatars, it's stupid for every website to host avatars. I think Libravatar got it right: your identity provider can host your avatar, and if it doesn't there's a centralized service as fallback.

[Changaco]

custom option reticketed as #202.