Plugin v2.x webhook URL syntax no longer supported

[mickmister]

Summary

When we introduced multiple Jira instance support in v3.0, we changed the webhook URL structure to include an identifier of the intended Jira instance. Updating the webhook URL was optional, since there was a check for backwards compatibility for the old webhook URL structure, though this backwards compatibility check regressed in v4.0.0.

As mentioned in the v2.2 tag's README, we previously a supported a syntax like:

https://SITEURL/plugins/jira/api/v2/webhook?secret=WEBHOOKSECRET

While it looks like this today:

https://SITEURL/plugins/jira/instance/INSTANCE_ID/api/v2/webhook?secret=WEBHOOKSECRET

The task here is to restore backwards compatibility for the 2.x webhook URL syntax. This will involve modifying the mux.Router to allow the /webhook and api/v2/webhook endpoints that do not have the instance/INSTANCE_ID part of the URL.

The below block which handles this case was in 3.x but is not in 4.0.0. Note the ResolveWebhookInstanceURL that is called here. This is what normalizes the URL to make the INSTANCE_ID optional:

mattermost-plugin-jira/server/http.go

Lines 100 to 110 in acca43d

	routeIncomingWebhook,
	routeOAuth1Complete,
	routeUserDisconnect,
	routeUserConnect,
	routeUserStart,
	routeAPISubscribeWebhook:
	callbackInstanceID, err = p.ResolveWebhookInstanceURL(instanceURL)
	if err != nil {
	return respondErr(w, http.StatusInternalServerError, err)
	}

[raghavaggarwal2308]

@mickmister Can we close this as done via #989