add slack verification

[itsthejoker]

Relevant issue: N/a

Description:

Need to actually verify that requests coming in on the slack endpoint actually come from slack. This is a security hole and also since I'm about to implement a command that modifies data, we need to be extra sure. The actual process for verifying is a pain in the butt, but I think that this is accurate.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [ ] Tests (if applicable)
• [x] Success Criteria Met (I hope)
• [ ] Inline Documentation
• [ ] Wiki Documentation (if applicable)

[itsthejoker]

Update: this does appear to actually work

[itsthejoker]

The replay prevention is straight from Slack's documentation, so it's something we can add a test for but beyond that I'm taking their word.