Ogr

- We have added `target_branch_head_commit` property to the `PullRequest`
class in `ogr` that allows you to get commit hash of the HEAD of the
target branch (i.e. base, where the changes are merged to).

- We have introduced a new function into `ogr` that allows you to get commit
SHA of the HEAD of the branch. (668)
- A list of Gitlab projects provided by `GitlabService.list_projects()` now
contains objects with additional metadata. (667)

- OGR now fully supports getting PR comments by its ID.

- Removal of features which have been marked as deprecated since `0.14.0`.
- Removal of renamed properties
- `Comment.comment` -> `Comment.body`
- `BasePullRequest.project` -> `BasePullRequest.target_project`
- Removal of methods for accessing issues or pull requests from `GitProject` class.
- String can no longer be used as commit status, `CommitStatus` is now required.
- `PullRequest` constructor has been refactored. In order to use static and offline
representation of a pull request, use `PullRequestReadOnly` instead.
- `GithubCheckRun.app` property has been added for accessing `GithubApp`.

- Ogr now catches internal exceptions from Gitlab and Github and converts them
to ogr exceptions, GitlabAPIException and GithubAPIException, respectively. A
new exception, OgrNetworkError, has been introduced for signalling situations
where a request could not be performed due to a network outage. (642)
- The documentation was converted to Google-style docstrings. (646)
- Releases and development builds of ogr are now built in copr projects
packit/packit-dev and packit/packit-releases. (644)

- New method to get pull request and issue comments by their comment ID on
GitHub and GitLab.