Add new event: ChangeLabeledEvent

[morucci]

Some projects use a Change workflow heavily based on "labels" to define Change states. For instance a project might define that a Change is in 'awaiting-review' or 'awaiting-author' state. (for instance: https://github.com/leanprover-community/mathlib)

By indexing such events in Monocle database it might be possible for a Monocle user and for that specific workflow (described above) to leverage some Monocle facilities to compute a waiting for review average time metric.

Monocle used to handle the most common's change attributes and events but as the LabeledEvent seems to exists on the GitHub GraphQL schema, it doesn't on GitLab.