Allow `allow_local_requests_from_hooks_and_services` to be set via API

Summary

After I update my GitLab to 11.5.3, the error 422 error Invalid url given occurred while I create webhook.

Steps to reproduce

1. Upgrate GitLab to 11.5.3
2. Create webhook

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behaviour, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug has been fixed in a more recent version)

What is the current bug behavior?

Create webhook failed.

What is the expected correct behavior?

Create webhook successfully.

Relevant logs and/or screenshots

POST http://192.168.0.33:10080/api/v4/projects/root/helloworld/hooks: 422 {error: Invalid url given}

Output of checks

This problem also happens on GitLab.com

Results of 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

(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

(If you can, link to the line of code that might be responsible for the problem)