atwho is still enabled for personal snippet comments form

Steps to reproduce:

1. On master, find a personal snippet, like https://dev.gitlab.org/snippets/160.
2. In the comment box, type # or ~ or another reference prefix.

Expected: the character is just inserted.

Actual: a loading spinner shows up, and there's a JS error in the console:

This is a bug in 9.2 as we didn't allow commenting on personal snippets before.