New version notifications - slack app

[accesstechnology-mike]

Webapp should notify users of new versions when discovered, include version number release date and link to main site (rather than direct) as we won't know the user's platform.

[accesstechnology-mike]

1. Direct link to slack notifcation app should be on the site
2. Can the slack app itself be stored on GitHub too? Or is it kept within Slack's ecosystem?