feat: add analytics module to handle /analytics endpoints

[karrui]

🗒 This PR is complete with the exception of integration tests for AnalyticsRouter, see note below in the test section.

Problem

This PR extracts out analytics related functions in various controllers and services and reorganises them into the modules/analytics directory.

Related to #144

Solution

Features:

• Add static findFormsWithSubsAbove method to SubmissionModel so it can be easily used by services
• Add static aggregateFormCount method to FormStatisticsTotalModel so it can be easily used by services
• Add AnalyticsService to retrieve user/submission/form counts from their various models.
• Add AnalyticsFactory to return correct function handler for retrieving form counts depending on whether aggregate-stats feature is enabled in the Feature Manager
• Add AnalyticsController controller handler for /analytics route
• Replace current /analytics route with the new AnalyticsRouter

Tests

• Add unit tests for analytics.service
• Add unit tests for analytics.factory
• Add unit tests for analytics.controller

This will be delayed until #371 is merged in as that contains code for generated an authenticated session (or should I cherry pick the appropriate commits? Scared that will scare ppl off with the commit size again)

• Add integration tests for analytics.routes

[liangyuanruo]

lgtm with minor suggestions!