fix(add-another): find labels of affixed text inputs

When a text input has affixes, it is wrapped in a `.govuk-input__wrapper` div, meaning its label is
no longer adjacent. This caused the Add anotehr component to fail, because it couldn't find the
input's label and update its for/id link.

I've added a new strategy for finding the label: if it's not the parent or adjacent, then JavaScript
identifies it by searching for a label whose `for` attribute is the ID of the original input.

This should not introduce any regressions because the previous strategies are still in place and
prioritised: the new strategy will only be used if nothing was found, which would have previously
resulted in the application crashing.

fixes #185

src/moj/components/add-another/README.md

@@ -0,0 +1,6 @@
+# Add another
+
+- [Guidance](https://moj-design-system.herokuapp.com/components/add-another)
+- [Preview](https://moj-frontend.herokuapp.com/components/add-another)
+
+## Arguments

src/moj/components/add-another/add-another.js

@@ -42,9 +42,13 @@ MOJFrontend.AddAnother.prototype.getNewItem = function() {
 
 MOJFrontend.AddAnother.prototype.updateAttributes = function(index, item) {
 	item.find('[data-name]').each(function(i, el) {
+    var originalId = el.id
+
 		el.name = $(el).attr('data-name').replace(/%index%/, index);
 		el.id = $(el).attr('data-id').replace(/%index%/, index);
-		($(el).siblings('label')[0] || $(el).parents('label')[0]).htmlFor = el.id;
+
+    var label = $(el).siblings('label')[0] || $(el).parents('label')[0] || item.find('[for="' + originalId + '"]')[0];
+		label.htmlFor = el.id;
 	});
 };