Dropdown: Make Identicons of a subgroup in the breadcrumb dropdown not take a whole line

What does this MR do?

This merge request fixes a display bug for subgroups without a custom icon in the breadcrumb dropdown

Before	After

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/51138 https://gitlab.com/gitlab-org/gitlab-ce/issues/54026 (duplicate)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #54026 (closed) and #51138 (closed)

cc: https://gitlab.com/gitlab-com/marketing/community-relations/contributor-program/gitlab-hackathon/q4-2018-hackathon/issues/1