Work only w/ timestamps in DateTimeRenderer

refs #6778
DateTimeRenderer: add 4 shortcuts

refs #6778
Add DateTimeRenderer's shortcuts as format helpers

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

This reverts commit 8ff7710a9b005bd16b06dc3b67a1766d8c66dfb1.
Revert "Use `dateTimeRenderer' helper in hostgroups list view"

This reverts commit 86d36c1024855d9b4a1188649cb04508ae225b56.
Revert "Deduplicate servicegroups list view's logic"

This reverts commit 47f96ff936a05e90919795f8ba4c163a9be4729c.
Revert "Deduplicate hostgroups list view's logic"

This reverts commit 7d7663a9af8fdfd8755ff1dba5d9ef8f07835084.
Icingacli: use DateTimeRenderer

refs #6778
ToggleInstanceFeaturesCommandForm: use new DateTimeRenderer helper

refs #6778
Use new DateTimeRenderer helper while displaying comments

refs #6778
Use new DateTimeRenderer helpers while displaying downtimes

refs #6778
Use new DateTimeRenderer helpers while displaying notifications

refs #6778
Use new DateTimeRenderer helper while listing hostgroups

refs #6778
Use new DateTimeRenderer helper while listing servicegroups

refs #6778
Use new DateTimeRenderer helper while listing hosts

refs #6778
Use new DateTimeRenderer helper while listing services

refs #6778
Use new DateTimeRenderer helpers while showing checkstatistics

refs #6778
Use new DateTimeRenderer helper while showing headers

refs #6778
Use new DateTimeRenderer helpers in process views

refs #6778
Drop obsolete helpers

refs #6778