Add support for email whitelist/blacklist to GitHub auth

If a site is configured for GitHub logins, _**and**_ has an email domain
whitelist, it's possible to get in a state where a new user is locked to
a non-whitelist email (their GitHub primary) even though they have an
alternate email that's on the whitelist.  In all cases, the GitHub
primary email is attempted first so that previously existing behavior
will be the default.

- Add whitelist/blacklist support to GithubAuthenticator (via
  EmailValidator)

- Add multiple email support GithubAuthenticator

- Add test specs for GithubAuthenticator

- Add authenticator-agnostic "none of your email addresses are allowed"
  error message.

lib/auth/github_authenticator.rb

@@ -1,18 +1,34 @@
+require_dependency 'has_errors'
+
 class Auth::GithubAuthenticator < Auth::Authenticator
 
   def name
     "github"
   end
 
+  class GithubEmailChecker
+    include ::HasErrors
+
+    def initialize(validator, email)
+      @validator = validator
+      @email = Email.downcase(email)
+    end
+
+    def valid?()
+      @validator.validate_each(self, :email, @email)
+      return errors.blank?
+    end
+
+  end
+
   def after_authenticate(auth_token)
     result = Auth::Result.new
 
     data = auth_token[:info]
+    result.username = screen_name = data[:nickname]
+    result.name = data[:name]
 
-    result.username = screen_name = data["nickname"]
-    result.email = email = data["email"]
-
-    github_user_id = auth_token["uid"]
+    github_user_id = auth_token[:uid]
 
     result.extra_data = {
       github_user_id: github_user_id,
@@ -20,20 +36,64 @@ class Auth::GithubAuthenticator < Auth::Authenticator
     }
 
     user_info = GithubUserInfo.find_by(github_user_id: github_user_id)
-    result.email_valid = !!data["email_verified"]
 
     if user_info
+      # If there's existing user info with the given GitHub ID, that's all we
+      # need to know.
       user = user_info.user
-    elsif result.email_valid && (user = User.find_by_email(email))
-      user_info = GithubUserInfo.create(
-          user_id: user.id,
-          screen_name: screen_name,
-          github_user_id: github_user_id
-      )
+      result.email = data[:email],
+      result.email_valid = !!data[:email_verified]
+    else
+      # Potentially use *any* of the emails from GitHub to find a match or
+      # register a new user, with preference given to the primary email.
+      all_emails = Array.new(auth_token[:extra][:all_emails])
+      all_emails.unshift({
+          :email => data[:email],
+          :verified => !!data[:email_verified]
+      })
+
+      # Only consider verified emails to match an existing user.  We don't want
+      # someone to be able to create a GitHub account with an unverified email
+      # in order to access someone else's Discourse account!
+      all_emails.each do |candidate|
+        if !!candidate[:verified] && (user = User.find_by_email(candidate[:email]))
+          result.email = candidate[:email]
+          result.email_valid = !!candidate[:verified]
+          GithubUserInfo.create(
+              user_id: user.id,
+              screen_name: screen_name,
+              github_user_id: github_user_id
+          )
+          break
+        end
+      end
+
+      # If we *still* don't have a user, check to see if there's an email that
+      # passes validation (this includes whitelist/blacklist filtering if any is
+      # configured).  When no whitelist/blacklist is in play, this will simply
+      # choose the primary email since it's at the front of the list.
+      if !user
+        validator = EmailValidator.new(attributes: :email)
+        found_email = false
+        all_emails.each do |candidate|
+          checker = GithubEmailChecker.new(validator, candidate[:email])
+          if checker.valid?
+            result.email = candidate[:email]
+            result.email_valid = !!candidate[:verified]
+            found_email = true
+            break
+          end
+        end
+
+        if !found_email
+          result.failed = true
+          escaped = Rack::Utils.escape_html(screen_name)
+          result.failed_reason = I18n.t("login.authenticator_error_no_valid_email", account: escaped)
+        end
+      end
     end
 
     result.user = user
-
     result
   end