Feature: Metrics calculation system overhaul

[SakuraIsayeki]

Context

When a user submits a post, this post is liable to change metrics on another user. This metric change is currently reactively "hand-calculated" by a service, and does not rely on any proactive method.

Problem

Any exception thrown between post submission and profile metrics update can and will invalidate any current metric, rendering it inaccurate.

Solution

The metrics calculation system should be changed for a proactive method (DB counts for example), to increase robustness in metrics readouts.