Add ProjectFeature check for feature flag

This will allow an explicitly-disabled feature flag to override a
feature being available for a project.

As an extreme example, we could quickly disable issues across all
projects at runtime by running `Feature.disable(:issues)`.

app/models/project_feature.rb

@@ -61,6 +61,9 @@ class ProjectFeature < ActiveRecord::Base
   end
 
   def feature_available?(feature, user)
+    # This feature might not be behind a feature flag at all, so default to true
+    return false unless ::Feature.enabled?(feature, user, default_enabled: true)
+
     get_permission(user, access_level(feature))
   end
 

spec/models/project_feature_spec.rb

@@ -82,6 +82,22 @@ describe ProjectFeature do
         end
       end
     end
+
+    context 'when feature is disabled by a feature flag' do
+      it 'returns false' do
+        stub_feature_flags(issues: false)
+
+        expect(project.feature_available?(:issues, user)).to eq(false)
+      end
+    end
+
+    context 'when feature is enabled by a feature flag' do
+      it 'returns true' do
+        stub_feature_flags(issues: true)
+
+        expect(project.feature_available?(:issues, user)).to eq(true)
+      end
+    end
   end
 
   context 'repository related features' do