Improve Concept Exercise: attack-of-the-trolls/flag-enums

This issue describes how to improve the `attack-of-the-trolls` concept exercise for the concept `flag-enums` for the C# track.

## Getting started

**Please please please read the docs before starting.** Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before creating the issue, please read up on the following documents:

- [The features of v3](https://github.com/exercism/v3/blob/master/docs/concept-exercises.md).
- [Rationale for v3](https://github.com/exercism/v3/blob/master/docs/rationale-for-v3.md).
- [What are concept exercise and how they are structured?](https://github.com/exercism/v3/blob/master/docs/features-of-v3.md)

Please also watch the following video:

- [The Anatomy of a Concept Exercise](https://www.youtube.com/watch?v=gkbBqd7hPrA).

## Goal

The Enums Advanced exercise should be lightly modified to require parsing an enum with `Enum.Parse`.
Consider adding the `generic-types` Concept as prerequisites if necessary (edit config.json and .meta/design.md).
Change the solution and test file and the .meta/Example.cs accordingly
Change the hints.md and instructions.md files accordingly

## Help

If you have any questions while improving the exercise, please post the questions as comments in this issue.