Add Signup route

We need something like `POST /signup` or `POST /users` to be able de self sign-up

Body should be like
```js
{
email: "string",
profileName: "string",
profileLocale: "string",
password: "string"
}
```

Verification should be made that e-mail is not already used and that given information are valid

User should be created with given informations along with an associated contributor, if `profileName` is given.

Validation code should be sent by e-mail, and the account should stay in `unconfirmed` state meanwhile.