add ref (resolved ref) into binder gallery

[MridulS]

resolved ref is added into launch events:

• add resolved ref information into launch events jupyterhub/binderhub#1111
• https://github.com/jupyterhub/binderhub/blob/master/binderhub/event-schemas/launch.json

but gallery is ignoring it while saving launches:

• https://notebooks.gesis.org/gallery/api/v1.0/launches/2019-06-19T12%3A00%3A00/2019-07-19T12%3A00%3A00

so we have to

• add ref into BinderLaunch model/table (models.py)
• create a database migration for the new field
• add ref into API (api.py)
• update mybinder_archives.py for ref

[Moved from GitLab]