Update remaining references to kinto-signer in repo

[grahamalama]

Though signer and changes were merged into the kinto-remote-settings plugin present in this repository, there are still a few references to the old kinto-signer plugin. We should update any references to kinto-signer to some other name that represents the signer functionality in the kinto-remote-settings plugin.

When we close this issue, we should also update references to kinto-signer in the kinto-admin repo, as there's at least one instance where we rely on the specific kinto-signer name.

[leplatrem]

note: The mentioned reference is the user id (plugin:kinto-signer) as the «author» of resources changes (switch of status in the collection metadata, etc.).