Add API route to register a new user

[AlphaHydrae]

The main added feature is the POST /api/users route. Additionally:

• The backend now has a generic mechanism to send emails (see
  development guide).
• In addition to what is required for registration, columns have been
  added to the users table to identify the authentication provider.
  Currently there is only the local provider, but in the future there
  may be Facebook, Twitter, etc.
• When a new user registers, the email is reserved until the
  registration process has been completed. However, after the expiration
  of the last generated OTP (the lifespan defaults to 2 hours), new
  registration requests will be accepted for the same email, replacing the
  incomplete registration (i.e. user) in the database.
• JWTs now have authorization scopes (see development guide). This is
  used in the context of user registration: a registration OTP is
  retrieved from the link sent in the registration email. This OTP allows
  a user to obtain a JWT with a special register scope. This JWT will
  allow to mark the user's email as verified (in the next PR).
• JWTs with infinite validity can now be generated (see POST /api/auth
  documentation).

The following code changes and refactorings have been made:

• The ECMAScript version in .eslintrc.json has been increased to
  ECMAScript 2018 to support the object spread operator.
• All mentions of "e-mail" have been replaced by "email".
• All getExpected* functions (e.g. getExpectedUser) that were in
  server/spec/utils.js have been moved to the respective
  server/spec/expectations/<model>.js file. This makes more sense as
  they are expectation-related.
• The file server/spec/utils.js has been split into several files in
  the new server/spec/utils directory. This was necessary to avoid a
  circular dependency.
• Test coverage for the files in server/api/auth and
  server/api/users has been brought to 100% excluding parts that will
  be completed when implementing PATCH /api/users/:id, as those
  features are critical to security.

The following changes have been made to the dependencies:

• https://www.npmjs.com/package/fast-glob replaces glob as it has a
  Promise-based API.
• fs-extra and handlebars are now production dependencies instead of
  development dependencies because they are used to load and parse email
  templates when starting the server.
• https://www.npmjs.com/package/js-yaml has been added to parse the YAML
  front matter of emails (see development guide).
• https://www.npmjs.com/package/query-string and
  https://www.npmjs.com/package/url-join have been added to facilitate
  building URLs (used in this PR to build the registration link).

Stories: TG-1, TG-2

[coveralls]

Coverage decreased (-0.6%) to 91.449% when pulling 4ad7b13 on story-2-user-registration into fd59ea8 on sprint-3-engage-in-an-action.

[saraheig]

GG