Add JSON field for arbitrary run data

[eyeseast]

I called it data because that's what it is. Not going to index this unless we think there's a good reason.

Closes #401

[duckduckgrayduck]

My open questions for Mitch:

• Do we want to standardize the naming and name this scratch too?
• Since this will primarily be used for the new Klaxon front-end and there are far more runs than events would limiting it to a URLField maybe be more storage friendly?
  I can imagine scenarios where unoptimized JSON blobs get stored and since the file size limit of JSON fields are much larger (iirc 256MB on Postgresql) this could grow fast

[eyeseast]

My thinking on naming: scratch implies that whatever's stored there can be thrown away or overwritten. In Klaxon, we overwrite the latest timestamp on each change. This field is for permanent storage.

I do think we should figure out how to put limits on this, though I need to look at what our options are for a JSON field. If we really wanted, we could even make it an HStore field.

[duckduckgrayduck]

That makes sense to me for naming