Fix label in related content form

It didn't have a `for` attribute and so it wasn't correctly associated
with its input. That means clicking on / touching the label didn't have
the effect of focusing on the field, and screen readers wouldn't
announce the label.

app/views/relationable/_form.html.erb

@@ -3,7 +3,7 @@
 </button>
 
 <%= form_tag related_contents_path, method: :post, id: "related_content", "data-toggler": "unused-toggle-class" do %>
-  <label><%= t("related_content.label") %></label>
+  <label for="url"><%= t("related_content.label") %></label>
 
   <p class="help-text" id="related_content_help_text">
     <%= t("related_content.help", models: t("related_content.content_title").values.to_sentence, org: setting["org_name"]) %>

spec/shared/system/relationable.rb

@@ -36,7 +36,7 @@ shared_examples "relationable" do |relationable_model_name|
     expect(page).to have_css ".add-related-content[aria-expanded='true']"
 
     within("#related_content") do
-      fill_in "url", with: "#{url + related1.url}"
+      fill_in "Link to related content", with: "#{url + related1.url}"
       click_button "Add"
     end
 
@@ -53,7 +53,7 @@ shared_examples "relationable" do |relationable_model_name|
     click_button "Add related content"
 
     within("#related_content") do
-      fill_in "url", with: "#{url + related2.url}"
+      fill_in "Link to related content", with: "#{url + related2.url}"
       click_button "Add"
     end
 
@@ -69,7 +69,7 @@ shared_examples "relationable" do |relationable_model_name|
     click_button "Add related content"
 
     within("#related_content") do
-      fill_in "url", with: "http://invalidurl.com"
+      fill_in "Link to related content", with: "http://invalidurl.com"
       click_button "Add"
     end
 
@@ -83,7 +83,7 @@ shared_examples "relationable" do |relationable_model_name|
     click_button "Add related content"
 
     within("#related_content") do
-      fill_in "url", with: url + relationable.url.to_s
+      fill_in "Link to related content", with: url + relationable.url.to_s
       click_button "Add"
     end