Missing labels / accessible names for inputs and select elements across admin

[aviannamiller]

Currently, there are missing labels for the following:

• Appearance -> Navigation: main_nav_checkboxes_# inputs
• Settings -> Element Sets -> Edit Element Set: elements-##-comment textarea inputs

There are missing accessible names for the following:

• Item Type -> Add (or Edit) Item Type -> "Existing": .existing-element-drop-down select elements