Warn users if creating a new source that has a similar name to an existing one

Users regularly create multiple sources for themselves. We should prompt users with an extra confirmation message if either of the following occur:

* the participant email is the same as a source already associated with the account
* the first name (e.g., `participant_name.split()[0]`) is `==` to the first name of another source on the account

A user should be allowed to proceed if they want.

The warning message should indicate that "We noticed you're creating a new source that may be similar or the same as an existing one. If the source created here really is the same human being as a prior source, then please consider assigning the sample to that source rather than creating a new one."

...or something like that