Disqus fails to load when the shortname includes a hyphen

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io

### What's the problem

#### Current behaviour

If your disqus shortname includes a hyphen (`-`), the comments will fail to load, yielding instead:

> We were unable to load Disqus. If you are a moderator please see our troubleshooting guide

### Provide steps to reproduce:

1. Create a disqus site with a shortname that has a hyphen, e.g.: "hackmd-disqus"
2. Try to add it to a note via `disqus: hackmd-disqus`
3. Load the note preview
4. See error