This adds FileAttachment.sha256_checksum, a SHA256 file content hash

of a file. We expose this attribute via the API so that callers can use

it to checksum file attachments. We generate and set this hash in our

upload file form, and in another spot where we create a file attachment for

the original version of binary files in the diff viewer. Even if these

two points don't cover all file attachment creation cases, the hash gets

set on demand whenever the sha256_checksum attribute is accessed.
This also adds a utility function for creating SHA256 hashes, and a

type hint to FileDiff.get_repository() that was useful for this work.
An upcoming change will point FileDiff.patched_sha256 and

FileDiff.orig_sha256 to the corresponding

FileAttachment.sha256_checksum for binary file diffs.

Ran unit tests.
Used in an upcoming RBTools change where I access the sha256_checksum

  attribute via the API.