Setting custom hooks in GitLab UI
### Description
As a GitLab admin of a large instance I would like the ability for users to set their own custom hooks in the UI rather than have it serviced and maintained by admin.
As a user of GitLab, I would like to be able to set custom hooks in the UI so it doesn't require admin intervention and having access to the server.
### Proposal
A setting in the project settings that allow users to drop in their custom hooks into the custom_hooks directory.
#### Overview
What is it? A custom hook field in the project settings
Why should someone use this feature? To display custom messages, etc... without admin intervention/oversight
What is the underlying (business) problem? That custom hooks can only be implemented by users with ssh access (admin).
How do you use this feature? By dropping in a file that drops it into the custom_hooks directory on the server.
#### Use cases
Who is this for? Provide one or more use cases.
Users that want to implement custom hooks in a self-service fashion. And admin that manage large instances that may not be able to support custom hooks due to demand and bandwidth restrictions.
### Feature checklist
Make sure these are completed before closing the issue,
with a link to the relevant commit.
- [ ] [Feature assurance](https://about.gitlab.com/handbook/product/#feature-assurance)
- [ ] Documentation
- [ ] Added to [features.yml](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/features.yml)
issue