The "Explore snippets" page loads very slowly with a large number of
snippets, due to the complexity of the authorization checks for project
snippets.

Since project snippets are of limited interest on the explore page, we
can restrict the query to personal snippets which are visible to the
current user.

We're also replacing the index on `snippets.project_id` with a combined
index on `(project_id, visibility_level)` to further speed up these
queries.