Add OpenID connect support

[leplatrem]

• Read identity providers from OpenID capability (root URL)
• Fetch ${idp}/.well-known/openid-configuration
• Start redirection to do login «dance»
• Read userinfo from ID token (display name top right menu)
• Put access token in session storage
• Use access token in Authorization: Bearer {} requests headers

Depends on Kinto/kinto#1425 (comment)

[leplatrem]

The kinto-oidc-demo demonstrates how to do it:
https://github.com/leplatrem/kinto-oidc-demo

[leplatrem]

Might be interesting to have this #331

[leplatrem]

Note from Julien:

On storing tokens in local storage: we're actively trying to define a
standard to follow in services, and so far it seems that tokens in local
storage with strong CSP to reduce the XSS surface is reasonable.

This might change over time (like, well, everything) but it should not
be blocking work. Just don't forget the CSP ;)

According to https://hacks.mozilla.org/2016/02/implementing-content-security-policy/ a <meta> tag is enough...

See also Kinto/kinto#1075