add info banners

[rtrzebinski]

On home page:

• display welcome banner for new user without any lessons subscribed or created, inform that user can either create new lesson subscribe one of available lessons

^^ hide once user created or subscribed a lesson

On home page and lesson page:

• display a info banner for new user, with at least one lesson (subscribed or created), without at least 2 exercises, about minimum required to start learning, and how to add exercises

• if user has at least 2 exercises - display banner with info that he can learn

^^ hide once user opened learning page at least once

On learning page:

• display banner with information about learning mode, hide either once result exist or manually (with 'x')