Upgrade TinyMCE to v5.1.0

Introduces new sticky toolbar option!

gui/public/tinymce/skins/content/dark/content.min.css

@@ -0,0 +1,7 @@
+/**
+ * Copyright (c) Tiny Technologies, Inc. All rights reserved.
+ * Licensed under the LGPL or a commercial license.
+ * For LGPL see License.txt in the project root for license information.
+ * For commercial licenses see https://www.tiny.cloud/
+ */
+body{background-color:#2f3742;color:#dfe0e4;font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Oxygen,Ubuntu,Cantarell,'Open Sans','Helvetica Neue',sans-serif;line-height:1.4;margin:1rem}a{color:#4099ff}table{border-collapse:collapse}table td,table th{border:1px solid #6d737b;padding:.4rem}figure{display:table;margin:1rem auto}figure figcaption{color:#8a8f97;display:block;margin-top:.25rem;text-align:center}hr{border-color:#6d737b;border-style:solid;border-width:1px 0 0 0}code{background-color:#6d737b;border-radius:3px;padding:.1rem .2rem}td[data-mce-selected],th[data-mce-selected]{color:#333}.mce-content-body:not([dir=rtl]) blockquote{border-left:2px solid #6d737b;margin-left:1.5rem;padding-left:1rem}.mce-content-body[dir=rtl] blockquote{border-right:2px solid #6d737b;margin-right:1.5rem;padding-right:1rem}