6. As a user, I want to be able to invite others to an existing shopping list

[fullybaked]

Summary

We want to allow users to invite others to existing shopping lists so they can manage them with friends or family. This would take the following steps:

1. User enters an existing user's email into a form
2. The shareList function in src/api/firebase.js is called with the appropriate inputs

Please note that an existing user is a user that has signed into the app before.

Acceptance criteria

•  The ManageList view shows a form that allows the user to enter an email to invite an existing user to a list, in addition to the form that allows them to add items to that list.
•  The input that accepts the email has a semantic label element associated with it
•  The user can submit this form with both the mouse and the Enter key
•  If the other user exists, the user is alerted that the list was shared
•  If the other user does not exist, the user is shown an error message that explains the problem

Notes

• You will know your <label> is correctly implemented if you click on the label and keyboard focus moves to the related input. Refer to the MDN docs on <label> for more info.
• Users will be able to submit the form with the Enter key if you listen for the right JavaScript event on the right element! If you're adding an onKeyDown listener anywhere, you might be overcomplicating things.
• Check the shareList function in src/api/firebase.js to get an idea of how you will know if a list was shared.