Add datetime helpers, reduce arrow usage to a minimum

[xmatthias]

Summary

Reduce arrow usage to a minimum in favor of internal datetime helpers (we always need the date in exactly the same format).
It'll also help should we need to make further changes in this regards.

Usage of datetime is not reduced by this, but will be replaced by dt_now() over time.

Arrow remains a dependency for now, but used only for humanization.
it's however not providing benefits to other areas - as datetime.now(timezone.utc) works perfectly fine to get a date (vs. arrow.utcnow().datetime.

Quick changelog

• don't use arrow throughout the code

[coveralls]

Coverage: 94.48% (-0.01%) from 94.493% when pulling 9d0f488 on feat/datetimehelpers into 707c674 on develop.