Use `dateTimeRenderer' helper in checkstatistics view

refs #6778
Clean up downtime components view's logic

refs #6778
Deduplicate hostgroups list view's logic

refs #6778
Deduplicate servicegroups list view's logic

refs #6778
Use `dateTimeRenderer' helper in hostgroups list view

refs #6778
Use `dateTimeRenderer' helper in servicegroups list view

refs #6778
Icinga Web view helpers: add TODO

refs #6778