Create tinymce6 skin to match the old tinymce4 one

[GuySartorelli]

The tinymce4 skin we are using is not at all compatible with tinymce6 and needs to be effectively remade from scratch.
There are instructions here for doing so: https://www.tiny.cloud/docs/tinymce/6/creating-a-skin/#creating-a-skin
Unfortunately the tool mentioned on that page isn't available for timymce6 so it has to be done manually.

Acceptance criteria

• The tinymce6 editor looks close enough to the how the tinymce4 one did

Notes:

• It does not need to look identical - people will know it's two majors ahead, so some change in appearance (different icons, etc) is to be expected
• The main goal is that it fits in seemlessly into the general content author experience and doesn't look out of place

PRs

• ENH Implement our tinymce skin for tinymce 6 #1371

[sabina-talipova]

TinyMCE 6 works and looks perfect!