Make the Slug header optional for the deposit server

Currently, the deposit server requires the Slug header, so the client generates one when needed.

However, as @douardda pointed out to me, the SWORD specification says it is optional:

The client MAY supply a Slug header providing a suggested identifier for the deposited content

and

Server implementations MUST adopt the behaviour and requirements in Section 9.7 of [AtomPub] with respect to the Slug header.

and from the AtomPub spec:

Slug is an HTTP entity-header whose presence in a POST to a Collection constitutes a request by the client to [...]

which implies optionally as well.

So we should make the server accept the absence of a Slug header and generate a slug on its own if it is missing. And then, we can remove the slug generation from the client, as it won't be needed anymore.

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Migrated from T2767 (view on Phabricator)