refactor(embeds): drop slideshare, use vimeo api, tighten CSP

The slideshare integration was broken for quite a while already,
as slideshare doesn't seem to have a good replacement, we're
dropping it in the same manner as speakerdeck was dropped some
time ago. This means the special syntax now just renders a plain
link. This commit gets rid of the vimdo oembed API too which
allowed JSONP injection. Instead we're using the normal vimeo
video metadata API.

Co-authored-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Erik Michelson <github@erik.michelson.eu>

public/docs/features.md

@@ -238,10 +238,6 @@ When you’re a carpenter making a beautiful chest of drawers, you’re not goin
 
 {%gist schacon/4277%}
 
-#### SlideShare
-
-{%slideshare briansolis/26-disruptive-technology-trends-2016-2018-56796196 %}
-
 #### PDF
 
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