don't allow posting in locked topics

lib/philomena/users/ability.ex

@@ -109,7 +109,8 @@ defimpl Canada.Can, for: [Atom, Philomena.Users.User] do
   def can?(_user, :show, %Topic{hidden_from_users: false}), do: true
   def can?(_user, :show, %Post{hidden_from_users: false}), do: true
   
-  # Edit posts
+  # Create and edit posts
+  def can?(_user, :create_post, %Topic{locked_at: nil, hidden_from_users: false}), do: true
   def can?(%User{id: id}, :edit, %Post{hidden_from_users: false, user_id: id}), do: true
 
   # View profile pages

lib/philomena_web/controllers/topic/post_controller.ex

@@ -13,7 +13,7 @@ defmodule PhilomenaWeb.Topic.PostController do
   plug :load_and_authorize_resource, model: Forum, id_field: "short_name", id_name: "forum_id", persisted: true
 
   plug :load_topic
-  plug PhilomenaWeb.CanaryMapPlug, create: :show, edit: :show, update: :show
+  plug PhilomenaWeb.CanaryMapPlug, create: :create_post, edit: :create_post, update: :create_post
   plug :authorize_resource, model: Topic, id_field: "slug", id_name: "topic_id", persisted: true
 
   plug PhilomenaWeb.CanaryMapPlug, edit: :edit, update: :edit

lib/philomena_web/templates/topic/show.html.slime

@@ -50,12 +50,16 @@ h1 = @topic.title
       = pagination
 
   / Post form
-  = if @topic.post_count < 200_000 do
-    = render PhilomenaWeb.Topic.PostView, "_form.html", conn: @conn, forum: @forum, topic: @topic, changeset: @changeset
-  - else
-    /h3 Okay, we're impressed
-    /p You're looking at a thread with over 200,000 posts in it!
-    /p For various reasons, we'd like to ask you to start a new topic.
+  = cond do
+    - @topic.post_count < 200_000 and can?(@conn, :create_post, @topic) ->
+      = render PhilomenaWeb.Topic.PostView, "_form.html", conn: @conn, forum: @forum, topic: @topic, changeset: @changeset
+
+    - @topic.post_count < 200_000 ->
+      h3 Okay, we're impressed
+      p You're looking at a thread with over 200,000 posts in it!
+      p For various reasons, we'd like to ask you to start a new topic.
+
+    - true ->
 
   / Mod tools
   /- if can? :assist, Topic