Add Google & Auth0 authentication

[eraviart]

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I added support for Auth0 authentication because this is an easy way to add login/password authentication to HackDash.

To implement it, I had to add two optional fields to providers: name and icon.

You can see it running on https://hackathon.ogpsummit.org (try "Email" authentication).

Caution: provider_id returned by Auth0 are strings instead of numbers. So I changed the type of the provider_id attribute of User but I didn't write the script to convert existing provider_id to strings.

[impronunciable]

@eraviart this is really cool! my only concern is, who is paying for the auth0 auth?

[eraviart]

My understanding of Aut0 site, is that the service is free until we reach 7000 active users per month.

Since we use Auth0 service only for email authentication, I believe I won't reach this limit, even for our "huge" hackathon :-)

[impronunciable]

@eraviart Awesome. Thank you very much. I'll take a look and merge if everything is okay.

Thank you very much for the contribution