Provide error message on duplicate RSVP

[luanzeba]

Problem statement

When a given user tries to RSVP to an event they’re already registered for we should display a flash message with the following content:

User has already registered for this event

Steps to reproduce

1. Start your Rails server with bin/dev command
2. Create an event at http://localhost:3000/events/new
3. Create a user at http://localhost:3000/users/new
4. Click RSVP on the event page twice

How to fix it

I have added unit tests to help guide you in the implementation in the duplicate_rsvp_message development branch.

I'd be happy to review your implementation when I have time. Please open a pull request following these steps:

1. Follow this guide to setup your forked repo
2. Checkout a development branch

git fetch upstream
git switch -c duplicate_rsvp_message upstream/duplicate_rsvp_message

3. Run all tests to see what's failing with bin/rails test
4. Make your changes to make tests pass
5. Open a pull request and tag me for review