Attach maturity timer to PR, not user

[Matt Cowley]

Feature description

Whilst this achieves the same thing at the end of the day, many users were confused by the maturity timer being attached to the user, where it wouldn't show until 4 PRs were completed.

We should give each PR its own timer so that once a user has submitted their first PR, they can see a timer.

The win logic remains the same here though, as they won't win until their fourth PR passes the maturity timer, which is how it works currently.