Need a Slack app workflow for when a new member joins the reviewers channel

[millerh1]

The workflow will be:

1. New reviewer is added to channel
2. Slack bot welcomes them with a message
3. Message prompts them to enter their github userid
4. They enter it and hit "Submit" -- this sends a POST request to the CRUD app
5. The CRUD app will enter the reviewer info into the DB
6. From this point onward, the reviewer can just interact with the slack channel like normal

If the user does fill out the form, they should be prompted again for each visit to the channel (@jmsdao is this feasible?)