V11: Update the rich text editor TinyMCE to v6 (#13013)

* update 'tinymce' from v4 to v6

* add/remove plugin & file changes

* remove unsupported plugins

* update changes to tinymce api

* bump tinymce to 6.2.0

* reinstate fastdev task

* update rte prevalues

* fix styling of tinymce inline mode

* document new files

* update builtin allowed elements, plugins and styles

* update upload_image_handler to new format

* set correct startup plugins and config

* remove old config for "Paste from Word" which is no longer supported

* update naming in default toolbar

* replace fire() with dispatch()

* support resizing of other elements besides objects, e.g. tables

* replace fire() with dispatch()

* change classes for fullscreen styling

* handle special parsing of AngularJS json response

* remove deprecated tinymce options

* use EditorOptions API to get settings

* send in correct toolbar config

* update icons to use tinymce's new icon pack

* remove unused functions

* configure custom styles

* update docs

* add initial toolbars to grid rte

* change htmlId selector to support html id's with numbers

* remove pintoolbar as it is not compatible with tiny 6

* register custom option maxImageSize through the OptionsManager

* remove custom option

* Revert "register custom option maxImageSize through the OptionsManager"

This reverts commit 342354d6cdacea67349d9d6d97b944999f8cd136.

* register custom option maxImageSize and fetch through EditorOptions

* add rounding to forced image size

* remove fullscreen plugin as it is not available in prod build anyway and it does not work

* update API for uploadImages() function

* update string extensions IsFullPath to support more filepaths with new built-in Path.IsPathFullyQualified

* resolve TODO to switch to Path.IsPathFullyQualified supported from .NET Standard 2.1

* ensure that files are copied from the TEMP folder in the web root

* add dragstart event to drag-events series

* optimise isMediaPickerEnabled function with newer array function 'includes'

* add TEMP folder to gitignore (fx fileuploads)

* update selector for "Insert Macro" button in rte test

* remove old documentation

* Use content root instead of web root for uploaded images

* test IsFullPath without tricky UNC paths that are not useful

Co-authored-by: Bjarke Berg <mail@bergmania.dk>

src/Umbraco.Core/Extensions/StringExtensions.cs

@@ -1326,8 +1326,6 @@ public static class StringExtensions
     /// </summary>
     /// <param name="path"></param>
     /// <returns></returns>
-    // From: http://stackoverflow.com/a/35046453/5018
-    // Updated from .NET 2.1+: https://stackoverflow.com/a/58250915
     public static bool IsFullPath(this string path) => Path.IsPathFullyQualified(path);
 
     // FORMAT STRINGS