[True Retention by Card Maturity] Investigate if overwriting todayStats is still necessary

[glutanimate]

We currently overwrite CollectionStats.todayStats in order to allow users to define a custom maturity ivl. This can cause conflicts with other add-ons that patch the same method.

We should investigate if this approach is actually still necessary. For starters, we should look into conditionally overwriting CollectionStats.todayStats only if a non-defaut MATURE_IVL is set.

(Thanks to Gustaf on tenderapp for the report)