Bug 17501: Rename Upload to Uploader

Bug 17501: Rename Upload to Uploader

Why? Koha::Uploader now only contains the actual CGI upload. The new name
better reflects its handler status.
Pragmatically, the difference between Uploaded and Uploader makes it
easier to specifically search for them in the codebase.

Test plan:
[1] Run t/db_dependent/Upload.t.
[2] Add an upload via the interface.
[3] Check the code:
    git grep "Koha::Upload;"
    git grep "Koha::Upload\->"

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>