Handle issues webhooks from GitHub

[joshsmith]

Problem

We need to write code that handles issues events in the CodeCorps.GitHub.Events.Issues module.

This should:

• Have handlers for the opened, edited, closed, and reopened actions.
• Does not yet handle the assigned, unassigned, labeled, unlabeled, milestoned, or demilestoned actions.
• Fetches the issue from GitHub.
• Checks whether the CodeCorps.Task exists for the GitHub task.
• Checks whether this is the last write (i.e. another event from some source did not happen since this event transpired). If this is the last write, this write wins and we should create or update the CodeCorps.Task.

While we can handle this later, some thoughts on assigned and unassigned actions:

• We'll need to simply handle the assignment and unassignment of the CodeCorps.Task based on the GitHub account that's connected to a user. No thoughts yet on how to handle certain edge cases here. Thus, this should wait to be handled later