editor: added plugin for tinymce css files

[0einstein0]

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Description

Partially Closes #581

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[kpsherva]

by adding this, we populate plugins for tinymce even for instances which do not have tinymce, I am not sure if it is the best approach

[max-moser]

Is this still relevant?

In the meanwhile, we've introduced the copy from/to instructions in webpack configs.
One of those is actually related to TinyMCE in Invenio-App-RDM.
Perhaps it should be implemented there instead.