Update to TinyMCE 6

[romintomasetti]

It seems that TinyMCE 6 is out.

I used

TINYMCE_JS_URL     = f'https://cdnjs.cloudflare.com/ajax/libs/tinymce/6.0.1/tinymce.min.js'

and it doesn't work. I get:

Uncaught TypeError: tinyMCE.editors is undefined
    initTinyMCE http://0.0.0.0:8000/static/django_tinymce/init_tinymce.js:39
    <anonymous> http://0.0.0.0:8000/static/django_tinymce/init_tinymce.js:59
    <anonymous> http://0.0.0.0:8000/static/django_tinymce/init_tinymce.js:58
[init_tinymce.js:39:11](http://0.0.0.0:8000/static/django_tinymce/init_tinymce.js)
    initTinyMCE http://0.0.0.0:8000/static/django_tinymce/init_tinymce.js:39
    <anonymous> http://0.0.0.0:8000/static/django_tinymce/init_tinymce.js:59
    forEach self-hosted:167
    <anonymous> http://0.0.0.0:8000/static/django_tinymce/init_tinymce.js:58

[claudep]

Not sure if it's enough for TinyMCE 6 compat, but the patch above should at least fix the reported error.