Impossible to unsubscribe from notifications on snippets
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### Summary
I keep getting email notifications for a snippet I commented on, and there's no way to unsubscribe from these notifications, either in notification emails or on the snippet page.
### Steps to reproduce
1. Comment on a snippet (creating one should also work). The one I commented on is in https://gitlab.com/gitlab-org/gitlab/-/snippets/. If you o it in a project, make sure it's not one where you have all notifications disabled.
2. Ask someone to make a comment on it.
3. Receive a notification email.
### Example Project
https://gitlab.com/gitlab-org/gitlab/-/snippets/
### What is the current *bug* behavior?
It's impossible to unsubscribe from notifications on snippets
### What is the expected *correct* behavior?
You unsubscribe from notifications on a snippet from the snippet page, email, and when defining the Custom notification setting.
### Relevant logs and/or screenshots
Email footer:
> 
### Output of checks
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This bug happens on GitLab.com.
### Possible fixes
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#### Results of GitLab environment info
<!-- Input any relevant GitLab environment information if needed. -->
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<summary>Expand for output related to GitLab environment info</summary>
<pre>
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)
(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)
</pre>
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#### Results of GitLab application Check
<!-- Input any relevant GitLab application check information if needed. -->
<details>
<summary>Expand for output related to the GitLab application check</summary>
<pre>
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:check SANITIZE=true`)
(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)
(we will only investigate if the tests are passing)
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issue