Adapts the existing post to a collection to allow only metadata deposit.
Endpoint:
```
POST /1/<collection>
HEADER: Content-type: application/atom+xml;type=entry
```
And the xml provided by the client contains either:
```
<swh:deposit>
<swh:reference>
<swh:object swhid="{swhid}" />
</swh:reference>
</swh:deposit>
```
or:
```
<swh:deposit>
<swh:reference>
<swh:origin url="{url}" />
</swh:reference>
</swh:deposit>
```
Any invalid swhid raises a 400 bad request. If everything passes, a 201
response with usual deposit receipt is received by the deposit user. The
deposit row is updated with status "done", complete_date and reception_date to
the same and current date, the swhid columns are updated as well if any.
Note:
This technically is reusing the existing code from the metadata update scenario.
The current metadata update still happens the same way as before.
There should be an extra diff to also relax the slug constraint (right now, we
are forced to pass it). I keep this modification out of this diff as I think
that will leave to more code moving around and that diff is quite long already.
Related to T2537