DEV: extract channel. joined_by method (#21752)

This only moves code around and doesn't change any behavior. This does two things:

1. Extracts the `channel.joined_by?` methods
2. Uses term "members" instead of "participants" for chat members

plugins/chat/app/models/chat/channel.rb

@@ -119,6 +119,14 @@ module Chat
       update_user_counts
     end
 
+    def joined_by?(user)
+      user.user_chat_channel_memberships.strict_loading.any? do |membership|
+        predicate = membership.chat_channel_id == id
+        predicate = predicate && membership.following if public_channel?
+        predicate
+      end
+    end
+
     def self.update_message_counts
       # NOTE: Chat::Channel#messages_count is not updated every time
       # a message is created or deleted in a channel, so it should not

plugins/chat/lib/chat/notifier.rb

@@ -158,20 +158,14 @@ module Chat
     end
 
     def group_users_to_notify(users)
-      potential_participants, unreachable =
+      potential_members, unreachable =
         users.partition { |user| user.guardian.can_join_chat_channel?(@chat_channel) }
 
-      participants, welcome_to_join =
-        potential_participants.partition do |participant|
-          participant.user_chat_channel_memberships.any? do |m|
-            predicate = m.chat_channel_id == @chat_channel.id
-            predicate = predicate && m.following == true if @chat_channel.public_channel?
-            predicate
-          end
-        end
+      members, welcome_to_join =
+        potential_members.partition { |user| @chat_channel.joined_by?(user) }
 
       {
-        already_participating: participants || [],
+        members: members || [],
         welcome_to_join: welcome_to_join || [],
         unreachable: unreachable || [],
       }
@@ -191,7 +185,7 @@ module Chat
 
       grouped = group_users_to_notify(direct_mentions)
 
-      to_notify[:direct_mentions] = grouped[:already_participating].map(&:id)
+      to_notify[:direct_mentions] = grouped[:members].map(&:id)
       inaccessible[:welcome_to_join] = grouped[:welcome_to_join]
       inaccessible[:unreachable] = grouped[:unreachable]
       already_covered_ids.concat(to_notify[:direct_mentions])
@@ -211,7 +205,7 @@ module Chat
       @parsed_mentions.groups_to_mention.each { |g| to_notify[g.name.downcase] = [] }
 
       grouped = group_users_to_notify(reached_by_group)
-      grouped[:already_participating].each do |user|
+      grouped[:members].each do |user|
         # When a user is a member of multiple mentioned groups,
         # the most far to the left should take precedence.
         ordered_group_names =