Add and apply Style/LineEndConcatenation rule

We were already using it in most places.

Note that enabling this rule means we've got to change a few lines in
order to follow the LineEndStringConcatenationIndentation rule. In the
link list tests, the easiest way to do so was to use heredoc instead,
which IMHO improves readability over the previous version.

spec/components/shared/link_list_component_spec.rb

@@ -18,9 +18,12 @@ describe Shared::LinkListComponent do
       ["Home", "/"], ["Info", "/info"], class: "menu"
     )
 
-    expect(page).to be_rendered with: '<ul class="menu">' + "\n" +
-                                      '<li><a href="/">Home</a></li>' + "\n" +
-                                      '<li><a href="/info">Info</a></li>' + "\n</ul>\n"
+    expect(page).to be_rendered with: <<~HTML
+      <ul class="menu">
+      <li><a href="/">Home</a></li>
+      <li><a href="/info">Info</a></li>
+      </ul>
+    HTML
   end
 
   it "accepts anchor tags" do
@@ -28,9 +31,12 @@ describe Shared::LinkListComponent do
       '<a href="/">Home</a>'.html_safe, ["Info", "/info"], class: "menu"
     )
 
-    expect(page).to be_rendered with: '<ul class="menu">' + "\n" +
-                                      '<li><a href="/">Home</a></li>' + "\n" +
-                                      '<li><a href="/info">Info</a></li>' + "\n</ul>\n"
+    expect(page).to be_rendered with: <<~HTML
+      <ul class="menu">
+      <li><a href="/">Home</a></li>
+      <li><a href="/info">Info</a></li>
+      </ul>
+    HTML
   end
 
   it "accepts options for links" do