Stop displaying version check data

Relates to #37

The goal of this MR is to remove the UI bits relating to version checks:

CurrentHostStat: Stop displaying version check data

This removes version check related data from being displayed in the current host stats. We also filter the CurrentHostStat table by stats that have a usage_data_recorded_at.

Remove version check views

Stop displaying version check views.

Add a simple stats page

This page is useful to test our later migrations, as the the number of CurrentHostStat and Hosts should converge to the same number after we done our data migrations.

Derive Versions on Hosts page from Usage Datum

Instead of querying the version checks, we are querying the associated Usage Datums.

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Screenshots

What	Before	After	Comment
Version Check Index		Deleted	/
Version Check Show		Deleted	/
Statistics Page	N/A		A quick new view to get the total number of host / current host stat counts
Current Host index			Removed the version_check_created_at column. Filtering by where.not(usage_data_recorded_at: nil)
Host Show			Removed links to version checks. Deriving the list of versions from usage_datum instead of version checks