Allow users to add collaborators who do not have shorebird accounts

[bryanoltman]

Description

To add a collaborator to an app, the collaborator must already have a shorebird account. While it is easy to create an account, it would be better if we allowed users to invite collaborators who have not yet signed up for Shorebird, as it is one less step and one less thing for our users to think about.

[bryanoltman]

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