Accepting an invitation link leads to a 500 error if user has an existing account. (BB-16099)

Currently the invitation link will only work if the user has yet to register a Bitbucket account. This shouldn't be the case - all users should be able to accept the invitation regardless if they have an account or not, with:

• users that have yet to have an account to be lead to registration page.
• users that have an account to be lead immediately to the team account page.