Keep stats per repo

[ccascone]

Is your feature request related to a problem? Please describe.
To facilitate debugging and administration, we need a way to answer questions like:

• For which repos are we checking CLAs?
• When was the last time a CLA was checked for a given repo?

Describe the solution you'd like
Create a new collection repoStats and instrument the functions to creare/update entries for each CLA validation event with:

• document ID: the repo ID (e.g. github.com/foo/bar)
• eventCount: cumulative number of events (can use firestore increment to make sure we update this field atomically)
• lastEventOn: timestamp of last event

Create an admin page that visualizes data from this collection.

Describe alternatives you've considered
For GitHub, we could check CLAM app stats if those are made available to developers. In any case, since we support both gerrit and github, it would be nice to see stats in the same place, i.e. the clam admin panel.

[teone]

@ccascone once we hace the repoStats collection I can take care of visualizing the stats