Put synced GitHub issues into the correct TaskList or archive it

[joshsmith]

Problem

We need to modify the GitHub sync so that when we create or update the Task we:

• associate tasks without pull requests and with status: "open" in the TaskList where inbox: true
• associate tasks with pull requests and with status: "open" in the TaskList where pull_requests: true
• associate task with status: "closed" in the TaskList where done: true and modified_at < 30 days
• archive task with status: "closed" where done: true and modified_at >= 30 days, and nullify any association with a TaskList

In order to do this, we'll probably want to modify the GitHub.Sync.Issue.Task.Changeset to:

• See if there is a GithubPullRequest for the given GithubIssue
• Move the TaskList |> Repo.get_by logic into a separate function that takes the changeset and the GithubIssue and determines the right logic per the above
• Pattern match against the status and modified_at timestamps in order to either set the archived field or set the task_list_id association

[joshsmith]

This is blocked by #1151 and #1152