Add trending petition journals

[cih]

This change adds trending petition journals for recording signature
counts at hourly intervals and returns them from the petition show
API endpoint as suggested in #458.

[pixeltrix]

@cih you can rebase against master and force push to rebuild this and it should work

[dom96]

Why is this stalled?

[pixeltrix]

@dom96 because the journal tables have always been a pain when we get a massive spike in demand like we saw with the Revoke Article 50 petition and the thought of adding more journals fills me with dread at the thought of having to resync them all. Not closed to the idea of providing this data just looking to see if there's a way we can make PostgreSQL do the heavy lifting.

[CodePint]

Heya, just thought I would share the project i'm working on to add full tracking to petitions by geography/locale.

https://github.com/CodePint/PetitionTrackerUK

Bulk of the work is done, but without optimisations you are correct that regular polling adds a large amount of DB load, I am working on a solution to this currently.

Likely solution will be:

• Base scenario of only creating a new record every hour for all petitions above 500 signatures
• Maintain a table of trending petitions which will be polled more regularly
• Some form of dataset reduction and archival after a month.

Ive got the bulk of the work done and a react frontend with chart.js hooked up. Tried only updating a locale if new signatures we're present in an effort to reduce storage, but unfortunately this just shifts the problem to long query times.