Can't invite user to be co-presenter on more than one proposal

[tylerdave]

While working on the fix for #53 I found another issue related to adding co-presenters. Once an email has been invited to be a co-presenter once, subsequent invitations result in an exception.

duplicate key value violates unique constraint "symposion_speakers_speaker_invite_email_key" DETAIL: Key (invite_email)=(user@example.com) already exists.