Support mobile document uploads

[rnicholus]

This adds support for mobile document uploads by exposing a file input in the postings section. This allows for easy upload of documents for a specific transaction and account.

I considered adding a file input to the Context dialog, but that proved to be a dead-end since the entry hash changes on file upload. In order to support that, there would either need to be a way to re-calculate the hash after file upload and then reload the context dialog without disturbing unsaved changes, or a way to send the file upload and changes to the transactions in a single atomic PUT request. Those are both possible, but probably not worth the effort.

This alternative implementation has a bit better UX in my opinion, as it allows to easily associate a document with an account simply by clicking the file upload button next to the posting entry.

closes #979

[rnicholus]

The development doc is either incomplete or out-of-date, so i'll need some assistance getting the build to pass here.