[signoff] Make it clearer when we only show "changes since the last signature"

[glasserc]

kinto-admin/src/plugins/signoff/sagas.js

Line 112 in fe81b91

// Fetch the source changes made since the last signature (since=0 means everything).

Otherwise, it can just seem like there's a bug where we don't show all the records for some reason.

[leplatrem]

Also, we should take into account the fact that collections can be resigned (source.last_modified > preview.last_modified), if that's relevant

[leplatrem]

I can't find where this applies. Do you recall @glasserc ?

[glasserc]

This might have gotten fixed by #936.

[glasserc]

We can't figure out where this issue was applied. It doesn't look like we filter the records to only show those since the last signature.