GitHub - Check for an existing webhook when subscribing to a repo

[aaronrothschild]

When a user create a subscription to a repo the plugin should check the repo (or parent Organization for Enterprise) to see if a webhook is in place to deliver the events

If no webhook is found on the GitHub side, an ephemeral post should occur to remind the user that a webhook must be set up. It can then post the url that must be used to deliver the events.

Note that this functionality is already implemented for GitLab and some of this implementation may be salvageable. cc @DHaussermann

[hanzei]

One thing to consider while implementing this is which token to use for fetching the information about what webhook to use.

[hanzei]

Requires #456 to be merged first.

[hanzei]

The token use to check check the webhook should be the one from the user executing the command. It them doesn't have the appropriate permissions, the check doesn't fail, it just isn't performed.

[sibasankarnayak]

@hanzei just wanted to check

will it be okey if we return the message when the subscription is successful and there is no webhook for repo/owner

Successfully subscribed to mattermost/mattermost-plugin-github ,
can configure the webhook to receive notification [here](https://localhost:8065/admin_console/plugins/plugin_github)

[sibasankarnayak]

@hanzei have implemented this feature in this way , let me know if you have any suggestion from your side