Apply Layout/LineLength rubocop rule

Note we're excluding a few files:

* Configuration files that weren't generated by us
* Migration files that weren't generated by us
* The Gemfile, since it includes an important comment that must be on
  the same line as the gem declaration
* The Budget::Stats class, since the heading statistics are a mess and
  having shorter lines would require a lot of refactoring

spec/system/proposals_spec.rb

@@ -505,7 +505,9 @@ describe "Proposals" do
 
   scenario "JS injection is prevented but autolinking is respected", :no_js do
     author = create(:user)
-    js_injection_string = "<script>alert('hey')</script> <a href=\"javascript:alert('surprise!')\">click me<a/> http://example.org"
+    js_injection_string = "<script>alert('hey')</script> " \
+                          "<a href=\"javascript:alert('surprise!')\">click me<a/> " \
+                          "http://example.org"
     login_as(author)
 
     visit new_proposal_path