Adding email signup and recovery

[ineiti]

This PR adds the possibility to create accounts linked to an email.

1. Setup - the system needs to be set up once with an account that is allowed to create new users
2. Signup - by sending an email, the service creates a new user and sends the signup link to the given email
3. Recover - once a user is signed up, they can request a mail to recover their account, and a recover link will be sent to the email stored on the ledger

This PR has three parts:

• updates to the byzcoin system for convenience
• adding needed functions to the personhood/user package
• adding service endpoints to the personhood service

Review request

1. Please start at looking at the description of the service itself: https://github.com/dedis/cothority/blob/email_recovery/personhood/README.md#email-signup-and-recovery a. Is it understandable? b. Does it sound good?
2. Look through the changed files in the darc/, byzcoin/, and external/ directories for obvious shortcomings
3. Look at the new service endpoints in personhood/service.go and follow through the code to see if it's understandable and doesn't have obvious errors. This should cover mostly new code, at least as long as it's in the personhood-directories.

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
10 Code Smells

No Coverage information
1.2% Duplication

[ineiti]

@tharvik I let you close the open conversations if you're happy with my comments / changes.