Advanced Search create FF to disable snippet_titles global search

Background

While debugging gitlab-com/gl-infra/production#15770 (closed) I found that global searches in the API are allowing snippet_titles scope. This scope should have a FF (ops type) added so we can control availability in GitLab.com

Proposal

• Create a new ops type FF (disabled by default)
• Make sure it's implemented for both SearchController and API::Search
• Add specs