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.

.rubocop.yml

@@ -573,6 +573,9 @@ Style/IdenticalConditionalBranches:
 Style/IfWithBooleanLiteralBranches:
   Enabled: true
 
+Style/LineEndConcatenation:
+  Enabled: true
+
 Style/MapToHash:
   Enabled: true
 

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

spec/system/admin/budget_investments_spec.rb

@@ -1797,8 +1797,8 @@ describe "Admin budget investments", :admin do
       columns_cookie = cookies.find { |cookie| cookie[:name] == "investments-columns" }
       cookie_value = columns_cookie[:value]
 
-      expect(cookie_value).to eq("id,title,supports,admin,valuator,geozone," +
-        "feasibility,price,valuation_finished,visible_to_valuators,selected,incompatible")
+      expect(cookie_value).to eq("id,title,supports,admin,valuator,geozone,feasibility,price," \
+                                 "valuation_finished,visible_to_valuators,selected,incompatible")
     end
 
     scenario "Use column selector to display visible columns" do
@@ -1847,8 +1847,8 @@ describe "Admin budget investments", :admin do
       columns_cookie = cookies.find { |cookie| cookie[:name] == "investments-columns" }
       cookie_value = columns_cookie[:value]
 
-      expect(cookie_value).to eq("id,supports,admin,geozone," +
-        "feasibility,valuation_finished,visible_to_valuators,selected,incompatible,author")
+      expect(cookie_value).to eq("id,supports,admin,geozone,feasibility,valuation_finished," \
+                                 "visible_to_valuators,selected,incompatible,author")
 
       visit admin_budget_budget_investments_path(budget)
 
@@ -1856,8 +1856,8 @@ describe "Admin budget investments", :admin do
       columns_cookie = cookies.find { |cookie| cookie[:name] == "investments-columns" }
       cookie_value = columns_cookie[:value]
 
-      expect(cookie_value).to eq("id,supports,admin,geozone,feasibility,valuation_finished," +
-        "visible_to_valuators,selected,incompatible,author")
+      expect(cookie_value).to eq("id,supports,admin,geozone,feasibility,valuation_finished," \
+                                 "visible_to_valuators,selected,incompatible,author")
     end
 
     scenario "Select an investment when some columns are not displayed" do