FEATURE: Allow using invites when DiscourseConnect SSO is enabled (#12419)

This PR allows invitations to be used when the DiscourseConnect SSO is enabled for a site (`enable_discourse_connect`) and local logins are disabled. Previously invites could not be accepted with SSO enabled simply because we did not have the code paths to handle that logic.

The invitation methods that are supported include:

* Inviting people to groups via email address
* Inviting people to topics via email address
* Using invitation links generated by the Invite Users UI in the /my/invited/pending route

The flow works like this:

1. User visits an invite URL
2. The normal invitation validations (redemptions/expiry) happen at that point
3. We store the invite key in a secure session
4. The user clicks "Accept Invitation and Continue" (see below)
5. The user is redirected to /session/sso then to the SSO provider URL then back to /session/sso_login
6. We retrieve the invite based on the invite key in secure session. We revalidate the invitation. We show an error to the user if it is not valid. An additional check here for invites with an email specified is to check the SSO email matches the invite email
7. If the invite is OK we create the user via the normal SSO methods
8. We redeem the invite and activate the user. We clear the invite key in secure session.
9. If the invite had a topic we redirect the user there, otherwise we redirect to /

Note that we decided for SSO-based invites the `must_approve_users` site setting is ignored, because the invite is a form of pre-approval, and because regular non-staff users cannot send out email invites or generally invite to the forum in this case.

Also deletes some group invite checks as per https://github.com/discourse/discourse/pull/12353

app/models/invite.rb

@@ -2,6 +2,8 @@
 
 class Invite < ActiveRecord::Base
   class UserExists < StandardError; end
+  class RedemptionFailed < StandardError; end
+  class ValidationFailed < StandardError; end
 
   include RateLimiter::OnCreateRecord
   include Trashable
@@ -31,7 +33,6 @@ class Invite < ActiveRecord::Base
   validates :email, email: true, allow_blank: true
   validate :ensure_max_redemptions_allowed
   validate :user_doesnt_already_exist
-  validate :ensure_no_invalid_email_invites
 
   before_create do
     self.invite_key ||= SecureRandom.hex
@@ -68,6 +69,10 @@ class Invite < ActiveRecord::Base
     email.blank?
   end
 
+  def redeemable?
+    !redeemed? && !expired? && !destroyed? && link_valid?
+  end
+
   def redeemed?
     if is_invite_link?
       redemption_count >= max_redemptions_allowed
@@ -163,20 +168,23 @@ class Invite < ActiveRecord::Base
   end
 
   def redeem(email: nil, username: nil, name: nil, password: nil, user_custom_fields: nil, ip_address: nil, session: nil)
-    if !expired? && !destroyed? && link_valid?
-      raise UserExists.new I18n.t("invite_link.email_taken") if is_invite_link? && UserEmail.exists?(email: email)
-      email = self.email if email.blank? && !is_invite_link?
-      InviteRedeemer.new(
-        invite: self,
-        email: email,
-        username: username,
-        name: name,
-        password: password,
-        user_custom_fields: user_custom_fields,
-        ip_address: ip_address,
-        session: session
-      ).redeem
+    return if !redeemable?
+
+    if is_invite_link? && UserEmail.exists?(email: email)
+      raise UserExists.new I18n.t("invite_link.email_taken")
     end
+
+    email = self.email if email.blank? && !is_invite_link?
+    InviteRedeemer.new(
+      invite: self,
+      email: email,
+      username: username,
+      name: name,
+      password: password,
+      user_custom_fields: user_custom_fields,
+      ip_address: ip_address,
+      session: session
+    ).redeem
   end
 
   def self.redeem_from_email(email)
@@ -254,14 +262,6 @@ class Invite < ActiveRecord::Base
       end
     end
   end
-
-  def ensure_no_invalid_email_invites
-    return if email.blank?
-
-    if SiteSetting.enable_discourse_connect?
-      errors.add(:email, I18n.t("invite.disabled_errors.discourse_connect_enabled"))
-    end
-  end
 end
 
 # == Schema Information