DEV: Various behind-the-scenes improvements to PresenceChannel (#14518)

- Allow the `/presence/get` endpoint to return multiple channels in a single request (limited to 50)
- When multiple presence channels are initialized in a single Ember runloop, batch them into a single GET request
- Introduce the `presence-pretender` to allow easy testing of PresenceChannel-related features
- Introduce a `use_cache` boolean (default true) on the the server-side PresenceChannel initializer. Useful during testing.

app/controllers/presence_controller.rb

@@ -4,22 +4,35 @@ class PresenceController < ApplicationController
   skip_before_action :check_xhr
   before_action :ensure_logged_in, only: [:update]
 
+  MAX_CHANNELS_PER_REQUEST ||= 50
+
   def get
-    name = params.require(:channel)
+    names = params.require(:channels)
+    raise Discourse::InvalidParameters.new(:channels) if !(names.is_a?(Array) && names.all? { |n| n.is_a? String })
 
-    begin
+    names.uniq!
+
+    raise Discourse::InvalidParameters.new("Too many channels") if names.length > MAX_CHANNELS_PER_REQUEST
+
+    user_group_ids = if current_user
+      GroupUser.where(user_id: current_user.id).pluck("group_id")
+    else
+      []
+    end
+
+    result = {}
+    names.each do |name|
       channel = PresenceChannel.new(name)
+      if channel.can_view?(user_id: current_user&.id, group_ids: user_group_ids)
+        result[name] = PresenceChannelStateSerializer.new(channel.state, root: nil)
+      else
+        result[name] = nil
+      end
     rescue PresenceChannel::NotFound
-      raise Discourse::NotFound
+      result[name] = nil
     end
 
-    if !channel.can_view?(user_id: current_user&.id)
-      # Do not reveal existence of channel
-      raise Discourse::NotFound
-    end
-
-    state = channel.state
-    render json: state, serializer: PresenceChannelStateSerializer, root: nil
+    render json: result
   end
 
   def update
@@ -39,7 +52,7 @@ class PresenceController < ApplicationController
       raise Discourse::InvalidParameters.new(:leave_channels)
     end
 
-    if present_channels && present_channels.length > 50
+    if present_channels && present_channels.length > MAX_CHANNELS_PER_REQUEST
       raise Discourse::InvalidParameters.new("Too many present_channels")
     end