Automatically Handling Prerequisites

[0x4007]

• There's situations where we price tasks that actually aren't ready to be worked on due to prerequisite tasks that need to be completed first.
• It could be useful to set the priority and time level but then use a new command like /requires <issue url>
• In this case, the price label can be removed, indicating that it is not ready to be worked on.
• When the prerequisite issue is merged, it would be great to add pricing again, which will automatically allow contributors to /start again.

This needs to listen to:

1. the issue.closed (as complete) webhook event, and then restore the latest price from the timeline.
2. the issues_comment.created webhook event, to parse the command.

[0x4007]

@sshivaditya2019 it is "editing" the spec even without posting relevance. This should not occur.