Block same-file reuploads

When uploading a file, there are a few ways of checking for and blocking
(or at least warning about) duplicate uploads already.

However, it occasionally seems to happen that files get uploaded twice.
The exact same file, usually - submitted at (almost) the exact same time
(possibly some error in whatever submits the file upload, but still)

Given 2 uploads at (almost) the exact same time, both of them are stored,
even if they are the exact same files.
The last upload also ends up with a `logging` entry with `log_page = 0`.

I don’t believe such upload should go through: if we do find that a file
is an exact duplicate of something that already exists, I don’t see any
reason it should go through.

Note that with this patch, it will become impossible to reupload a file
with the exact same hash (which was possible before.)
If we still want to allow same-file reuploads while also blocking these
kind of race-condition same-uploads, we could make the check more strict
(e.g. also check timestamps, or check if page already exists, or …)

Bug: T158480
Change-Id: I76cbd2c64c3b893997f1f85974d6f82cbfe121e1