Don’t use $yellow to indicate local environment

It clashes with the new `$govuk-focus-colour` now. This commit changes
it to half way between `govuk-colour("dark-grey")` (`#505a5f`) and
`govuk-colour("mid-grey")` (`#b1b4b6`) from the Design System. Dark was
too dark and mid was too light.

It also adds a line of JS to let us easily switch the header to blue by
clicking on it, which is useful for taking screenshots etc.

app/assets/javascripts/main.js

@@ -20,3 +20,7 @@ $(() => GOVUK.modules.start());
 $(() => $('.error-message').eq(0).parent('label').next('input').trigger('focus'));
 
 $(() => $('.banner-dangerous').eq(0).trigger('focus'));
+
+$(() => $('.govuk-header__container').on('click', function() {
+  $(this).css('border-color', '#005ea5');
+}));