Fix invalid HTML in related content button

A button cannot be inside an anchor tag, and it might confuse some
browsers or screen readers.

We're also making it clear in the tests that the intention is to use a
button there by using `click_button` instead of `click_on` since the
latter also clicks on links.

app/views/relationable/_related_content.html.erb

@@ -5,11 +5,9 @@
       <%= t("related_content.title") %>&nbsp;<span>(<%= relationable.relationed_contents.count %>)</span>
     </h2>
     <% if current_user %>
-      <a>
-        <button type="button" data-toggle="related_content" class="add-related-content" id="add-related-content">
-          <%= t("related_content.add") %>
-        </button>
-      </a>
+      <button type="button" data-toggle="related_content" class="add-related-content" id="add-related-content">
+        <%= t("related_content.add") %>
+      </button>
     <% end %>
   </div>
 

spec/shared/system/relationable.rb

@@ -30,7 +30,7 @@ shared_examples "relationable" do |relationable_model_name|
 
     expect(page).not_to have_selector("#related_content")
 
-    click_on("Add related content")
+    click_button "Add related content"
 
     within("#related_content") do
       fill_in "url", with: "#{url + related1.url}"
@@ -47,7 +47,7 @@ shared_examples "relationable" do |relationable_model_name|
       expect(page).to have_content(relationable.title)
     end
 
-    click_on("Add related content")
+    click_button "Add related content"
 
     within("#related_content") do
       fill_in "url", with: "#{url + related2.url}"
@@ -63,7 +63,7 @@ shared_examples "relationable" do |relationable_model_name|
     login_as(user)
     visit relationable.url
 
-    click_on("Add related content")
+    click_button "Add related content"
 
     within("#related_content") do
       fill_in "url", with: "http://invalidurl.com"
@@ -77,7 +77,7 @@ shared_examples "relationable" do |relationable_model_name|
     login_as(user)
     visit relationable.url
 
-    click_on("Add related content")
+    click_button "Add related content"
 
     within("#related_content") do
       fill_in "url", with: url + relationable.url.to_s