Better handling for empty values in some attributes

[jagthedrummer]

Fixes #247

The attribute display for date, date_and_time, and boolean_button did some unexpected things if the attribute in question was empty.

• boolean_button would display a label, but no value.
• date and date_and_time would display the word "Never" but without a label

All of the other attribute type don't display anything if their value is empty.

In order to allow apps to continue to show those values even if they are empty (but now with a label) those attribute partials now accept a display_when_blank option. It can be used like this:

<%= render 'shared/attributes/date', attribute: :date_field_value, display_when_blank: true %>

Before:

After, default behavior:

After, passing display_when_blank: true: