Use JSON instead of FormData

[saifahn]

Overview

The new API routes use JSON instead of FormData - I thought it would be easier to test and we also don't have images so we don't need FormData...

Affected files:

• [x] SharedTxnCreateForm
• [x] SharedTxnFormbase
• [x] SharedTxnReadUpdateForm
• [x] TxnCreateForm
• [x] TxnFormBase
• [x] TxnReadUpdateForm

Although, perhaps I'm doing something completely wrong - the POST /trackers route doesn't seem to be accepting the payload correctly hmmm

[saifahn]

The stuff about the trackers route - it was because I wasn't doing JSON.parse in the handler - this has been updated.

One thing was that the req._setBody method on the mock requests in the tests didn't make it easy to set JSON there, so I just assigned it with req.body =