Add an option to display section names in Section link block

Manual testing Create a course. Turn editing on and add a Section links block. Confirm that "Display section name" setting is set to No. Save changes. Confirm , that the block displays only the list of section numbers. Configure "Section links" block setting, set the "Display section name" setting to Yes and save changes. Confirm , that the block displays the list of sections with section name: Navigate to Site admin. Search for "showsectionname" setting and set it to Yes. Create a new course. Turn editing on and add a Section links block. Confirm , that the block honor "showsectionname" admin setting and displays section names straight away.