Add feature flag list command

doc/administration/troubleshooting/gitlab_rails_cheat_sheet.md

@@ -126,6 +126,18 @@ Benchmark.bm do |x|
 end
 ```
 
+## Feature flags
+
+### Show all feature flags that are enabled
+
+```ruby
+# Regular output
+Feature.all
+
+# Nice output
+Feature.all.map {|f| [f.name, f.state]}
+```
+
 ## Command Line
 
 ### Check the GitLab version fast