IssueComment.ChangesetBuilder does not require a payload

[begedin]

Problem

Right now, we pass in both the github api payload as well as a GithubComment into the changeset builder.

In reality, all we need is the GithubComment, since it contains the exact information we need to create a Comment.

• modify Adapters.Comment.to_comment/1 to accept a GithubComment struct (we can use Map.from_struct/1 and MapUtils.keys_to_string/1 for adapting it
• modify IssueComment.ChangesetBuilder.build_changeset/4 into build_changeset/3 by removing the payload attribute

This would simplify the code, and potentially allow us to decouple further in the future.