Replace relative imports for imports outside of the immediate module

[jamesbiggs]

Following a discussion on the developer catch-up, we decided it would be good practice to only use relative imports within the immediate module, e.g.

in etna.articles.blocks.py, this is fine:
from .models import ArticlePage

but we shouldn't use them like this:
from ..media.blocks import MediaBlock

and should instead be writing:
from etna.media.blocks import MediaBlock

It adds unnecessary additional cognitive load. The two apps are not immediately related to one another - etna.media can exist without etna.articles (in theory). So there is a chance that the path may change. It also makes it easier to trace where you are importing things from, and makes it clear to a developer trying to understand your code.

"Explicit is better than implicit."