Upgrade tinymce (#18107)

* Upgrade tinymce

* Omit 'skin_url' instead of setting it to false

The option expect a string value. Instead of setting it to 'false' we
can just completly omit it.
Otherwise we get the following warning:
> Invalid value passed for the skin_url option. The value must be a string.

Although not explicitly mentioned in the migration doc, the stricter check / warning has
probably been introduced with v6.

* Disable pasting of images

From
https://www.tiny.cloud/docs/tinymce/6/migration-from-5x/#plugins-paste:

> A paste default has changed. The paste_data_images option now defaults to true.
> When paste was a plugin, this option was, by default, set to false, which prevented images being pasted from the local machine

* Update toolbar option names

---------

Co-authored-by: Pascal Jufer <pascal-jufer@bluewin.ch>

app/src/interfaces/input-rich-text-html/tinymce-overrides.css

@@ -405,6 +405,11 @@ body.dark .tox .tox-toolbar__overflow {
 	visibility: hidden;
 }
 
+.tox:not(.tox-tinymce-inline) .tox-editor-header {
+	padding: 0;
+	box-shadow: none;
+}
+
 @media screen and (max-width: 767px) {
 	.tox .tox-dialog__body-nav-item {
 		text-align: center;