139: Disabled multimedia embedding

This change disables multimedia embedding:

- In comments and comments replies.
- In new submissions.
- In comment & submission preview

And it's all toggle-able via an envvar, except for the JS bits,
but I linked those to the github issue, so should be easy to find
in the future.

The way it works is:

- removes markdown image/video syntax,
  eg. `![](https://example.org/someimage.jpg)` into ``
- changes link text into anchors, eg.
  `https://example.org/someimage.jpg` into
  `[https://example.org/someimage.jpg](https://example.org/someimage.jpg)`
- removes html img/video/audio tags, eg.
  `<img href="https://example.org/someimage.jpg" />` into ``
- when embedding gifs via the giphy modal in "new submission", it will
  insert only an anchor to the gif
- when attaching an image, it will upload the image, then add only an
  anchor to the post/comment body

I tested this manually, but not sure if I got all the test cases. What I
checked was:

- create comment w/ image/video/audio media using markdown -> success
- create comment reply w/ image/video/audio media using markdown ->
  success
- create comment w/ link to img/imgur/youtube/audio -> success
- create comment w/ attachment -> success
- create comment reply w/ attachment -> success
- create comment w/ img/video tag -> success
- create comment reply w/ image/video tag -> success
- create post submission w/ image/video/media using markdown -> success
- create post submission w/ link to img/imgur/youtube/audio -> success
- create post submission w/ attachment -> success
- create post submission w/ giphy gif -> success

Also, updated the formatting page.

Co-authored-by: Ben Rog-Wilhelm <zorba-github@pavlovian.net>

files/templates/userpage.html

@@ -698,7 +698,7 @@
 {% endif %}
 
 <script src="/assets/js/vendor/purify.min.js?v=251"></script>
-<script src="/assets/js/vendor/marked.min.js?v=251"></script>
+<script src="/assets/js/vendor/marked.min.js?v=252"></script>
 <script src="/assets/js/marked.custom.js?v=251"></script>
 
 {% endblock %}