Add an option to filter strings by creation date

[ItielMaN]

This can be useful when you have lots of untranslated & unreviewd stings, but you just got a notification about new strings which you want to check out, and you don't want to look after each and every untranslated string to find which of them are the new ones.
In my case this would literally save me 10min~ each time new strings are added.

Quoting mathjazz in Pontoon channel on matrix:

Maybe also sorting by creation date would be useful
Or the ability to have all strings imported at the same time organized in groups