Issues.ChangesetBuilder does not require a payload

[begedin]

Problem

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

In reality, all we need is the GithubIssue, since it contains the exact information we need to create a Task from it.

• modify Adapters.Issue.to_task/1 to accept a GithubIssue struct (we can use Map.from_struct/1 and MapUtils.keys_to_string/1 for adapting it
• modify Issues.ChangesetBuilder.build_changeset/5 into build_changeset/4 by removing the payload attribute

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