Add pipeline syntax check when pages.publish and pages.pages.publish both specified

Summary

Add pipeline syntax check when pages:publish and pages:pages.publish both specified

Steps to reproduce

Inside pipeline editor:

Example Project

What is the current bug behavior?

No syntax error show in pipeline editor.

What is the expected correct behavior?

This should be caught in pipeline editor and only one value is allowed.

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