Guest users can't toggle notifications on an issue

Summary

Steps to reproduce

• Be added as a Guest to a project
• Go to an issue (which was not created by you)
• Try to turn on the notifications switch

Example Project

What is the current bug behavior?

The switch is greyed out, so you can't toggle it.

What is the expected correct behavior?

It should work.

Relevant logs and/or screenshots

1. Having someone else mention you, causes the switch to change to active, but then you, similarly, can't deactivate it.
2. Collapsing the sidebar and clicking the notifications button causes the error "Something went wrong while setting issue notifications." to show up.

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

(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`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(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)

Possible fixes