Move default projects features to admin web UI

Many admin-level settings have been moved from gitlab.yml/gitlab.rb to /admin/application_settings.

One set of settings that has not been made available in the admin web UI is gitlab_default_projects_features_(issues|merge_requests|wiki|snippets|container_registry). The corresponding visibility settings are already there, so it seems like this would clean up this facet of administration settings.

In gitlab-ce#23242, it was discovered that these settings didn't actually work. It was speculated that one reason was because the available settings had changed, but gitlab.rb had gone stale. This would be a reason to move it to the UI.

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Full backstory:
I have a user trying to import a project, but is getting the following error:

Error importing repository into xxx - Failed to replace merge_requests because one or more of the new records could not be saved. Target project has disabled merge requests

This seems to be related to gitlab-ce#31310 but as a workaround, I planned to just temporarily change the default projects settings globally for him. But then I realized this would require gitlab-ctl reconfigure, rather than just a simple UI change.