Notify a user when someone shares a project with them in private mode

[srish]

We now have a feature that allows users to share their project with another user in private mode #37. However, there are a couple of outstanding bugs #470 #454.

Right now, there is no way for a user to know if someone shares a project with them unless a communication happens outside ZubHub.

To Reproduce
You will need two accounts to test this feature. Log in with the first account and execute the following steps:

1. Try editing a project that you have created
2. Scroll to the bottom of the project form
3. Change the publish setting to 'preview'
4. Enter a valid username
5. Hit "Edit Project"

Log in with the second account. Only when you go to http://zubhub.unstructured.studio/creators/first_username you see the project in private mode. It would be nice to receive a notification about it.

[srish]

Closing this as we are considering hiding the sharing of a project with a specific user feature.