Bug 31558: Fix image drag and drop in TinyMCE

Bug 26949 upgraded JS lib TinyMCE to 5.9.2

This version has a known issue on images drag and drop :
https://stackoverflow.com/questions/64782955/tinymce-inline-drag-and-drop-image-upload-not-working

Test plan :
1) Go to Tools > HTML customizations
2) Create a new entry (not using text editor)
3) Drag and drop an image
=> Without patch you see an error message from TinyMCE 'Dropped file type is not supported'
=> With patch : It works !

Signed-off-by: David Nind <david@davidnind.com>

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit d0f09eaa64)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 278cd8b827)
Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>

koha-tmpl/intranet-tmpl/prog/en/modules/tools/additional-contents.tt

@@ -635,6 +635,7 @@
                 force_p_newlines : false,
                 forced_root_block : '',
                 branding : false,
+                block_unsupported_drop : false,
                 relative_urls : false,
                 content_css : "[% interface | html %]/[% theme | html %]/css/tinymce.css",
                 menubar : "file edit view insert format tools table",