FIX: Various invite system fixes (#13003)

* FIX: Ensure the same email cannot be invited twice

When creating a new invite with a duplicated email, the old invite will
be updated and returned. When updating an invite with a duplicated email
address, an error will be returned.

* FIX: not Ember helper does not exist

* FIX: Sync can_invite_to_forum? and can_invite_to?

The two methods should perform the same basic set of checks, such as
check must_approve_users site setting.

Ideally, one of the methods would call the other one or be merged and
that will happen in the future.

* FIX: Show invite to group if user is group owner

app/assets/javascripts/discourse/app/controllers/create-invite.js

@@ -151,6 +151,11 @@ export default Controller.extend(
           });
     },
 
+    @discourseComputed("currentUser.staff", "currentUser.groups")
+    canInviteToGroup(staff, groups) {
+      return staff || groups.any((g) => g.owner);
+    },
+
     @discourseComputed("type", "buffered.email")
     disabled(type, email) {
       if (type === "email") {

app/assets/javascripts/discourse/app/templates/modal/create-invite.hbs

@@ -83,7 +83,7 @@
     {{/if}}
 
     {{#if showAdvanced}}
-      {{#if currentUser.staff}}
+      {{#if canInviteToGroup}}
         <div class="input-group invite-to-groups">
           <label>{{i18n "user.invited.invite.add_to_groups"}}</label>
           {{group-chooser

app/assets/javascripts/discourse/app/templates/modal/share-topic.hbs

@@ -51,7 +51,7 @@
           {{d-button
             icon="check"
             class="btn-primary"
-            disabled=(not users)
+            disabled=(if users false true)
             action=(action "notifyUsers")
           }}
         </div>