Git LFS: validate upload size and SHA256 in Rails

When accepting an LFS upload, GitLab must verify that the SHA256 and size of the uploaded file are what the client claims them to be. These checks are currently implemented in Workhorse. This commit duplicates the checks in Rails, so that we can later remove them from Workhorse.

The changes in the controller itself are not very complicated. Most of the MR is about fixing tests that used to craft invalid requests, bypassing Workhorse. These requests must now have the right OID and size attributes in both their URL's and their headers.

The MR for removing the Workhorse version of the check is !86154 (merged).