Add format and type fields to Submission model

[TimJentzsch]

• format: Image/Video/Audio
• type: Twitter, Facebook, etc.

This data would need to be extracted from the transcription header.

It would allow us to generate stats based on the types, as well as to guarantee checks when a volunteer transcribes a new post type.

Challenges:

• Inconsistent header formats (e.g. [1/2] or (1/2) for multi-comment transcriptions)
• Should the types be grouped? (e.g. "Facebook" and "Facebook Comment", capitalization, etc.)
• What to do with old transcriptions with a misplaced header?