Account migration command

[itsthejoker]

Relevant issue: Closes #165

Description:

Adds a Slack command to migrate gamma between users.

Checklist:

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

[itsthejoker]

I have 100% test coverage on this -- going to merge so that I can test in slack. No other functionality should be impacted by this.