Fix gitlab:web_hook tasks

What does this MR do?

Fixes the gitlab:web_hook:rm task, and performs a number of small cleanups in the add and list tasks too. Adds tests for all three tasks.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22199 broke the gitlab:web_hook:rm task because we can no longer search for hooks by URL. There is a badly-performing workaround that we can do, where we scan the whole database (or a namespace-delimited subset thereof), which this MR represents.

Since these rake tasks are very old (introduced in 2013, not updated significantly since), they do a bunch of things in odd ways. I've cleaned up some of this, for instance, removing support for projects that are in no namespace at all.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #54975 (closed)