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/routes/comments.py

@@ -232,7 +232,10 @@ def api_comment(v):
 							requests.post(f'https://api.cloudflare.com/client/v4/zones/{CF_ZONE}/purge_cache', headers=CF_HEADERS, data={'files': [f"https://{request.host}/assets/images/badges/{badge.id}.webp"]}, timeout=5)
 						except Exception as e:
 							return {"error": str(e)}, 400
-				body += f"\n\n![]({image})"
+				if app.config['MULTIMEDIA_EMBEDDING_ENABLED']:
+					body += f"\n\n![]({image})"
+				else:
+					body += f'\n\n<a href="{image}">{image}</a>'
 			elif file.content_type.startswith('video/'):
 				file.save("video.mp4")
 				with open("video.mp4", 'rb') as f:
@@ -244,7 +247,10 @@ def api_comment(v):
 						if error == 'File exceeds max duration': error += ' (60 seconds)'
 						return {"error": error}, 400
 				if url.endswith('.'): url += 'mp4'
-				body += f"\n\n{url}"
+				if app.config['MULTIMEDIA_EMBEDDING_ENABLED']:
+					body += f"\n\n{url}"
+				else:
+					body += f'\n\n<a href="{url}">{url}</a>'
 			else: return {"error": "Image/Video files only"}, 400
 
 	body_html = sanitize(body, comment=True)