Prevent someone from creating a billion inactive users

[alexgleason]

Anyone can invite other users to manage their garden or plot by typing in their email address. This needs some limits, because:

1. This actually creates a user profile in the database as a result. Maybe it shouldn't do this, and instead create an "Invitation" or something. Adding a bunch of empty users is not semantically helpful and it makes me nervous that it could be exploited somehow (since users are directly related to authorization).

2. It sounds out an email to entered address, which could affect the reputation of gardenhub.io over time. We don't want anyone to consider this "spam".