Clone dashboard endpoint missing .yml errors

Summary

When making POST request with valid parametrs to /*namespace_id/:project_id/-/performance_monitoring/dashboards(.:format) 500 error is returned.

Steps to reproduce

This endpoint hasn't any FE usage yet. To reproduce it one has to use rest client (eg: postman)

1. Sign in to some account
2. Nagivate to existing project
3. From page source find authenticity_token
4. Construct valid request based on docummentation
5. 500 is returned

Example Project

Any project to which developer has push rights can be used to reproduce issue

What is the current bug behavior?

Error is raised

What is the expected correct behavior?

New commit with newly created .yml file is pushed to appointed branch.

Relevant logs and/or screenshots

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1120244/?query=is:unresolved

Output of checks

(If you are reporting a bug on GitLab.com, write: 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

Using absolute path to template files instead of relative one.