[add collaborators] existing users added via different email

[smkraus]

If an existing user gets invited via an email address that's not the one she uses for Detective.io there's no way to "transfer" this invitation to her existing account.

It would be ideal to:

• add a line in the invitation mail for un-registered users that says: "If you already have an account click here to add this investigation".
• possibly: add a link saying "Got an invite, but already have an account?" in the signup dialogue, whenever a "registration token" is present

Users should be led to a new dialogue where they can enter their user name to add it to the topic.