Store information about platform used when crowdsourcing content

[nya-elimu]

It would be useful to know if a contributor is using the website or the Crowdsource app when contributing/peer-reviewing content.

Add enum to separate the two.

• [x] Create PLATFORM enum
• [ ] Display platform at /content/multimedia/audio/list
• [ ] Display platform at /content/multimedia/audio/edit/<id>
• [ ] Add platform to peer-review events.

---

• Depends on #1265