Support pushing of feature flags to the frontend

What does this MR do?

This adds a method to Gitlab::GonHelper called push_frontend_feature_flag. This method can be used to easily expose the state of a feature flag to Javascript code. For example, using this method we may write the following controller code:

before_action do
  push_frontend_feature_flag(:vim_bindings)
end

def index
  # ...
end

def edit
  # ...
end

In Javascript we can then check the state of the flag as follows:

if ( gon.features.vimBindings ) {
  // ...
}

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/release/framework/issues/17

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the style guides