Use Setting.site_title value for og:site_name occurrences (#1194)

* Add helper method to return Setting.site_title

* Use site_title helper in application layout

* Use site_title value for og:site_name

spec/helpers/site_title_helper_spec.rb

@@ -0,0 +1,15 @@
+require "rails_helper"
+
+describe "site_title" do
+  it "Uses the Setting.site_title value when it exists" do
+    Setting.site_title = "New site title"
+
+    expect(helper.site_title).to eq "New site title"
+  end
+
+  it "returns empty string when Setting.site_title is nil" do
+    Setting.site_title = nil
+
+    expect(helper.site_title).to eq ""
+  end
+end