'projects#show' to supply links to next/previous project

app/controllers/projects_controller.rb
- Extract get_all_projects() from index().
- Modify show() to prepare @next_id and @prev_id.
app/views/projects/_project.html.erb
- Add links to next/previous project.
config/locales/ja.yml
- Add "next" and "prev" under "helpers.link".