Filter "All GitLab" / "My GitLab"

Description

When searching, it'd be nice to be able to switch between results for the whole of GitLab and only the projects and groups you're a part of. Kind of like the search in a Mac, where you can select "This Mac" or "This Folder"

Proposal

Add new options to the search scope dropdowns: Your groups and Your projects. Selecting them will search any project or group you belong to. Selecting Any group or Any project will search the whole instance.

Group filtering	Project filtering

Central issue gitlab-ce#18877