Prefer `start_with?` over `starts_with?`

Use parentheses for method call with arguments.
Reduce method length of ReadBoardMenu
Add util methods to ReadBoardMenu

- `selected_completable_board?`
- `selected_readable_board?`
Rename `read_post` to `read_post_helper`

Follow the convention of Rails. There is
`app/helpers/application_helper.rb`.
Update documentation for ReadPostHelper
Keep line length to 80