Add success or error feedback to `Join` link

[anikarni]

When the user clicks on the join link for a campaign, it should give a success message if added or a fail message if the extension is not installed.

[ProjectOak]

I've already added a dummy success state to see how it looks. There is no logic currently, no check for success or erro, so it will only show success for now, even if an error occurs. I'll add the error state and try to create the logic for it.

[ProjectOak]

Hey @jkarni, I've added the content that you had created and sent over email. Take a look at the page, it should better reflect the project now.

You'll also notice that I've removed the "how to section". Later on I will change that and bring back that section with the proper instructions.

[anikarni]

@ProjectOak , on the success message, I moved it to the plugin, so that we can only show it if it's actually a success. Will try to figure out a way to show an error if the plugin is not installed.