Fix loading static files with CachedStaticFilesStorage outside CMS

If HTML field is used outside the plugin then `CKEDITOR_BASEPATH` is not set and if CachedStaticFilesStorage is used then static files can't be loaded.

I moved setting `CKEDITOR_BASEPATH` into the widget not in the plugin. The only difference with previous solution is that I don't set the URL schema (because in the widget there is no access to current `request` instance). I think that this will not be a problem because browsers are made to handle this. If for some reason someone has a problem with that it can set the full URL in project's setting by setting `TEXT_CKEDITOR_BASE_PATH` to the right value.