posts: show takedown page for banned artists and posts.

Show a "This page has been removed because of a takedown request" error when
an unauthorized user searches for a banned tag, or tries to view a banned post.

app/controllers/application_controller.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class ApplicationController < ActionController::Base
+  class PageRemovedError < StandardError; end
+
   include Pundit::Authorization
   helper_method :search_params, :permitted_attributes
 
@@ -136,6 +138,8 @@ class ApplicationController < ActionController::Base
       render_error_page(422, exception, message: exception.message)
     when RateLimiter::RateLimitError
       render_error_page(429, exception, message: "Rate limit exceeded. You're doing that too fast")
+    when PageRemovedError
+      render_error_page(451, exception, template: "static/page_removed_error", message: "This page has been removed because of a takedown request")
     when Rack::Timeout::RequestTimeoutException
       render_error_page(500, exception, message: "Your request took too long to complete and was canceled.")
     when NotImplementedError

app/controllers/artists_controller.rb

@@ -40,6 +40,7 @@ class ArtistsController < ApplicationController
 
   def show
     @artist = authorize Artist.find(params[:id])
+    raise PageRemovedError if request.format.html? && @artist.is_banned? && !policy(@artist).can_view_banned?
     respond_with(@artist)
   end
 

app/controllers/posts_controller.rb

@@ -18,6 +18,8 @@ class PostsController < ApplicationController
       query = "#{post_set.normalized_query.to_s} random:#{post_set.per_page}".strip
       redirect_to posts_path(tags: query, page: params[:page], limit: params[:limit], format: request.format.symbol)
     else
+      raise PageRemovedError if request.format.html? && post_set.banned_artist?
+
       @preview_size = params[:size].presence || cookies[:post_preview_size].presence || PostGalleryComponent::DEFAULT_SIZE
       @posts = authorize post_set.posts, policy_class: PostPolicy
       respond_with(@posts) do |format|
@@ -28,6 +30,7 @@ class PostsController < ApplicationController
 
   def show
     @post = authorize Post.eager_load(:uploader, :media_asset).find(params[:id])
+    raise PageRemovedError if request.format.html? && @post.banblocked?(CurrentUser.user)
 
     if request.format.html?
       include_deleted = @post.is_deleted? || (@post.parent_id.present? && @post.parent.is_deleted?) || CurrentUser.user.show_deleted_children?

app/controllers/wiki_pages_controller.rb

@@ -31,6 +31,7 @@ class WikiPagesController < ApplicationController
 
   def show
     @wiki_page, found_by = WikiPage.find_by_id_or_title(params[:id])
+    raise PageRemovedError if request.format.html? && @wiki_page&.artist.present? && @wiki_page.artist.is_banned? && !policy(@wiki_page.artist).can_view_banned?
 
     if request.format.html? && @wiki_page.blank? && found_by == :title
       @wiki_page = WikiPage.new(title: params[:id])