Add logic to the internal api to create and store in the DB new pull request
When a repo is brand new, the start_id of a pull request might be null
Fix typo in a comment
Store the project from which to pull the changes when creating a pull request
Adjust the new_pull_request API method to account for the repo_from as well
Create the get_pull_request API method to retrieve pull requests from the DB
Specify relations on the PullRequest table
Have the Repository.path include the username in the output if repo is a fork

This simplifies things as we no longer have to consider the username
when loading a repository from the file system, we can just use the
information from the database.