Refactor swh-deposit to split the SWORD impl and SWH logic

swh-deposit mixes its home-grown SWORD implementations (client & server) with SWH logic, which makes it hard to read and maintain.

We should break the server into several components:

1. SWORD server
2. admin interface
3. server-side SWH logic (or move it all to the loader?)

And the client into:

1. SWORD client (or use the client written by Cottage Lab)
2. client-side SWH logic

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Migrated from T3814 (view on Phabricator)