Fix top-area inconsistent capitalization

What does this MR do?

Changes top-area navigation capitalization in /dashboard/activity and /dashboard/snippets to match the capitalization with the rest of links in the platform. More specifically, it changes Your Projects to Your projects, Starred Projects to Starred projects, Your Snippets to Your snippets and Explore Snippets to Explore snippets.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

To keep consistent capitalization across all navigation links, especially in links with two words (or more) like Your projects.

What are the relevant issue numbers?

No relevant issues. Initially mentioned in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16204.