Add `in this group` option to search dropdown where not present

Problem

When searching inside of a project from the top bar, in this group is not an available scope for the search. This means that if a user is looking at a project and then looking for results from their group they're not able to find results without searching and then adjusting the scope.

Proposal

The search dropdown should also provide a in this group option when searching from a project:

<TERM> in this project
<TERM> in this group
<TERM> in all GitLab

This will allow users to always scope searches that make the most sense for the results they're looking for.

Mockup