Kinto2xml

- Fix XML exporter on missing blockID. (63)

**Bug fixes**
- Allow the importer to work even without the permission to create collections. (56)
- Use `PUT` instead of `PATCH` to fully overwrite the destination with the source (58)

**New features**
- Retry downloading the XML in case it fails the first time. (50)

**Bug fix**
- Remove redundant footer when generating pages. (51)
- Allow the importer to work even without the permission to create buckets. (53)

**Internal changes**
- Improve the way the Makefile works (52)

**New features**
- Add a configurable signoff workflow to-review → to-sign (48)

**Bug fix**
- session_kwargs is not accessible anymore with last kinto-http client release. (45)

- Add the `blockpages-generator` CLI tool (43)