Develop: Determine the "sender" of each mail piece

As a user (especially one with a vision impairment), I want to be able to see the "Sender" for each physical mail image.

Acceptance Criteria

Sender field is saved to the MailPiece upon creation in the mail_fetcher class.
Some MailPieces come with an actual Sender as HTML text above the image. If available, this should be used as the Sender rather than using OCR on the image.
If no text Sender is supplied, the Sender for each MailPiece is determined via Google Cloud Vision
- Note: this may be pretty challenging to have accurate, as the sender doesn't always appear in the same spot and can be a logo.
If no sender can be determined, "Unknown Sender" should be saved instead
Document any relevant content in the Programmer guide (draft thoughts of usage and importantly the "Why")
Updates SRS and STP (and other docs if necessary)
Add/Update relevant unit tests