feat: support tracking stable branches

Closes #13. This PR implements tracking for stable branches. It includes the following changes: - Branches are slightly re-ordered (now it's staging > staging-next > master > ,,,) to better represent the staging workflow. - PRs targeting the stable branches are now correctly tracked, including stable staging PRs like the one mentioned in #13. - This is enabled by looking at the `head` and `base` fields of the API json responses, held in the `PullRequest` struct. Adding these fields is technically a breaking change for the lib crate, but really no one should be constructing these responses by hand and the fields are clearly non-exhaustive, so we take the chance to add the `#[non_exhaustive]` marker to the relevant structs. - Along the way, we add some pagination parameters in the API url for it to return less irrelevant information. This is a hack which _may_ speed up the responses and its processing. This is the first commit and it's not exactly related to the others so it can be dropped independently if it feels too unnecessarily hacky.