Avoid throwing abort in after_create method

What does this MR do and why?

Previously check_repository_absence! called throw :abort if it detected that the repository already existed. However, as discussed in https://github.com/rails/rails/issues/33192 Rails only supports throwing this signal in before_ callbacks.

Fix this by raising ActiveRecord::RecordInvalid if the project repository already exists. This should avoid "unhandled exception" errors in RSpec when this issue occurs.