Introduce present_on_default_branch:boolean to Vulnerability model

Why are we doing this work?

1. We have more than one source-of-truth for the Vulnerability state
2. It prevents us from building Vulnerability audit trail feature
3. It's unnecessarily complex

See #323129 (closed) and https://docs.google.com/document/d/1se3qnTH3bOuL9j-A4Kyu_au6eZlzU1zuvGzbOUj_fAw/preview for more details.

Implementation plan

1. database Create a migration that add present_on_default_branch:boolean column to vulnerabilities table with default value of true