DEV: introduce new API to look up dynamic site setting

This removes all uses of both `send` and `public_send` from consumers of
SiteSetting and instead introduces a `get` helper for dynamic lookup

This leads to much cleaner and safer code long term as we are always explicit
to test that a site setting is really there before sending an arbitrary
string to the class

It also removes a couple of risky stubs from the auth provider test

spec/components/plugin/instance_spec.rb

@@ -130,10 +130,11 @@ describe Plugin::Instance do
   end
 
   it 'patches the enabled? function for auth_providers if not defined' do
-    SiteSetting.stubs(:ubuntu_login_enabled).returns(false)
-
     plugin = Plugin::Instance.new
 
+    # lets piggy back on another boolean setting, so we don't dirty our SiteSetting object
+    SiteSetting.enable_badges = false
+
     # No enabled_site_setting
     authenticator = Auth::Authenticator.new
     plugin.auth_provider(authenticator: authenticator)
@@ -143,26 +144,28 @@ describe Plugin::Instance do
     # With enabled site setting
     plugin = Plugin::Instance.new
     authenticator = Auth::Authenticator.new
-    plugin.auth_provider(enabled_setting: 'ubuntu_login_enabled', authenticator: authenticator)
+    plugin.auth_provider(enabled_setting: 'enable_badges', authenticator: authenticator)
     plugin.notify_before_auth
     expect(authenticator.enabled?).to eq(false)
 
     # Defines own method
     plugin = Plugin::Instance.new
-    SiteSetting.stubs(:ubuntu_login_enabled).returns(true)
+
+    SiteSetting.enable_badges = true
     authenticator = Class.new(Auth::Authenticator) do
       def enabled?
         false
       end
     end.new
-    plugin.auth_provider(enabled_setting: 'ubuntu_login_enabled', authenticator: authenticator)
+    plugin.auth_provider(enabled_setting: 'enable_badges', authenticator: authenticator)
     plugin.notify_before_auth
     expect(authenticator.enabled?).to eq(false)
   end
 
   context "activate!" do
     before do
-      SiteSetting.stubs(:ubuntu_login_enabled).returns(false)
+      # lets piggy back on another boolean setting, so we don't dirty our SiteSetting object
+      SiteSetting.enable_badges = false
     end
 
     it "can activate plugins correctly" do

spec/components/site_setting_extension_spec.rb

@@ -532,6 +532,23 @@ describe SiteSettingExtension do
     end
   end
 
+  describe ".get" do
+    before do
+      settings.setting(:title, "Discourse v1")
+      settings.refresh!
+    end
+
+    it "works correctly" do
+      expect {
+        settings.get("frogs_in_africa")
+      }.to raise_error(Discourse::InvalidParameters)
+
+      expect(settings.get(:title)).to eq("Discourse v1")
+      expect(settings.get("title")).to eq("Discourse v1")
+    end
+
+  end
+
   describe ".set_and_log" do
     before do
       settings.setting(:s3_secret_access_key, "old_secret_key", secret: true)