Introduce __lte=now() instead of isnull=False

[aniav]

In #249 there is a suggestion to move from isnull=False to __lte=now(). I have done this and slightly changed the text in English and Polish version. I need someone to look at the Ukrainian version and tell me what change should I make to the one sentence that has been changed.

Previously there was:

You can also get a list of all published posts. We do it by filtering all the posts that have published_date:

Now we have:

You can also get a list of all published posts. We do it by filtering all the posts that have published_date set in the past:

In Ukrainian the sentence is currently:

Також можна отримати список усіх опублікованих постів. Зробимо це відфільтрувавши усі пости, що мають published_date:

[aniav]

@chupakabra can you help me with that? Should be an easy one ;)

[aniav]

Ukrainian version updated thanks to @ruslansavenok. Woohoo! :)