Add custom post type for webhook posts, in order to comment, edit, and close issues/PRs #618

At the moment, there are no actions shown on posts created by the plugin's bot. In Slack's example shown below, there are three actions:

• Comment
• Edit
• Close

The task here is to implement a new custom post type, which will render these actions at the bottom of the post for the MM webapp.

For the "edit issue" feature, there is already an existing feature to create a GitHub issue, so we would reuse the same modal, but have it opened by this new action button inside the post.

Shown below are the Comment, Edit and Close buttons on a post and an example of how the Close dialog look in Slack