Fix #5227: Error on note previews due to URL length limit

Allow either GET or POST for /note_previews.

app/controllers/note_previews_controller.rb

@@ -11,4 +11,6 @@ class NotePreviewsController < ApplicationController
       end
     end
   end
+
+  alias_method :create, :show
 end

app/javascript/src/javascripts/notes.js

@@ -526,7 +526,7 @@ class Note {
 
     async preview_text(text) {
       this.display_text("Loading...");
-      let response = await $.getJSON("/note_previews", { body: text });
+      let response = await $.post("/note_previews", { body: text });
 
       this.display_text(response.body);
       this.initialize();

config/routes.rb

@@ -172,7 +172,7 @@ Rails.application.routes.draw do
     end
   end
   resources :note_versions, :only => [:index, :show]
-  resource :note_previews, :only => [:show]
+  resource :note_previews, only: [:create, :show]
   resource :password_reset, only: [:create, :show]
   resources :pixiv_ugoira_frame_data, only: [:index]
   resources :pools do