Fix Slack integration

[jpd236]

For example:

• Slackbot should announce when puzzles are solved
• Relevant links should be posted in each Slack channel description (puzzle, spreadsheet, video call). I think this is already implemented and just broke due to the integration breaking, but if it doesn't, we can fork that into its own bug.

[jpd236]

I brought up a fresh instance on Heroku with only minor changes needed to get it up and running, and everything appears to be working fine with Slack, as far as I can tell. New puzzle announcements are happening, as are solve announcements. Puzzle channels have the link to the puzzle/spreadsheet posted (not the video chat, though).

In configuring Slack I noticed that:

• Some of the scopes listed in the README no longer existed
• There is some confusion around which OAuth credential to use. I think the intended one is the bot credential (beginning with "xoxb").

I've cleaned up the README a bit to reflect this, and I guess we'll need to test to confirm.

[jpd236]

I replaced the bot instance with my own and updated the credentials, and everything seems to be in order.

Video chat links were intentionally removed in https://github.com/jpd236/Big-Board/commit/52fd026d604cbf5aa725e9ddabfb7f1fbb623e82 before the last hunt, I think because we decided to use Slack huddles instead.