Update the Slack App Manifest to include the new URL for Interactivity

#432 added the ability to handle interactivity payloads from Slack Block interactive elements. 

Users who set up the app outside of SNDevs should be able to use the interactivity functionality when they load the Slack App Manifest, and as such the manifest needs to be updated.

The URL can be found in the app or in the PR linked at the start.