FEATURE: New share topic modal (#12804)

The old share modal used to host both share and invite functionality,
under two tabs. The new "Share Topic" modal can be used only for
sharing, but has a link to the invite modal.

Among the sharing methods, there is also "Notify" which points out
that existing users will simply be notified (this was not clear
before). Staff members can notify as many users as they want, but
regular users are restricted to one at a time, no more than
max_topic_invitations_per_day. The user will not receive another
notification if they have been notified of the same topic in past hour.

The "Create Invite" modal also suffered some changes: the two radio
boxes for selecting the type (invite or email) have been replaced by a
single checkbox (is email?) and then the two labels about emails have
been replaced by a single one, some fields were reordered and the
advanced options toggle was moved to the bottom right of the modal.

app/models/topic.rb

@@ -1683,6 +1683,27 @@ class Topic < ActiveRecord::Base
     email_addresses.to_a
   end
 
+  def create_invite_notification!(target_user, notification_type, username)
+    target_user.notifications.create!(
+      notification_type: notification_type,
+      topic_id: self.id,
+      post_number: 1,
+      data: {
+        topic_title: self.title,
+        display_username: username
+      }.to_json
+    )
+  end
+
+  def rate_limit_topic_invitation(invited_by)
+    RateLimiter.new(
+      invited_by,
+      "topic-invitations-per-day",
+      SiteSetting.max_topic_invitations_per_day,
+      1.day.to_i
+    ).performed!
+  end
+
   private
 
   def invite_to_private_message(invited_by, target_user, guardian)
@@ -1711,7 +1732,7 @@ class Topic < ActiveRecord::Base
     Topic.transaction do
       rate_limit_topic_invitation(invited_by)
 
-      if group_ids
+      if group_ids.present?
         (
           self.category.groups.where(id: group_ids).where(automatic: false) -
           target_user.groups.where(automatic: false)
@@ -1743,29 +1764,6 @@ class Topic < ActiveRecord::Base
   def apply_per_day_rate_limit_for(key, method_name)
     RateLimiter.new(user, "#{key}-per-day", SiteSetting.get(method_name), 1.day.to_i)
   end
-
-  def create_invite_notification!(target_user, notification_type, username)
-    target_user.notifications.create!(
-      notification_type: notification_type,
-      topic_id: self.id,
-      post_number: 1,
-      data: {
-        topic_title: self.title,
-        display_username: username
-      }.to_json
-    )
-  end
-
-  def rate_limit_topic_invitation(invited_by)
-    RateLimiter.new(
-      invited_by,
-      "topic-invitations-per-day",
-      SiteSetting.max_topic_invitations_per_day,
-      1.day.to_i
-    ).performed!
-
-    true
-  end
 end
 
 # == Schema Information