Project owner is not able to update or delete pages using Pages API

Summary

Project owner is not able to update or delete pages using Pages API. It is allowed from UI.

Steps to reproduce

1. Create a GitLab Pages website from a project template.
2. Wait for the pipeline to finish.
3. Try to perform Patch or Delete operation using Pages API.
4. User gets {"message":"403 Forbidden"}

Example Project

What is the current bug behavior?

User gets {"message":"403 Forbidden"}

What is the expected correct behavior?

The owner of the project should be able to perform the operation.

Relevant logs and/or screenshots

Output of checks

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