Add fallback text for summary list values

[peteryates]

Sometimes across projects we end up with 'or' statements within views so if a attribute is missing we display some relevant text instead, like this:

summary_list_row.with_value(text: object.stuff || "Not set")

It's a bit untidy and can get repetitive.

Perhaps if we allowed a fallback_text option that could have a default, we could remove some of this duplication and noise. When used on its own it might look like this:

summary_list_row.with_value(text: object.stuff, fallback_text: "Not set")

But with a default it's much cleaner.

# in config/components.rb
Govuk::Components.configure do |conf|
  default_summary_list_fallback_text: "Not set"
end

# in your view
summary_list_row.with_value(text: object.stuff)

Here, like we've done elsewhere, we could disable it on an ad hoc basis with fallback_text: false.