Save the branch when creating a new pull-request that's stored in the DB
When creating a new pull request, specify to which branch the pull-request should be made
Specify which fields are unique in a pull-request
Add a `check_pull_request` method to the internal API

This is meant to check if there is already an existing pull-request when
trying to create a new one.
Use the check_pull_request method to check if a pull-request exists before creating it