Add default home page content (in-app screen after registration)

[jryans]

As part of the next release, Riot bot will be disabled, and this means at the moment new users see the blank home page inside the app, which is not the most welcoming experience.

To address this, we want to revive the in-app home page (not to be confused with the "welcome page" which at least in code is the one outside the app before sign in) with some basic content (see "Latest" area in Figma, final subtitle text TBD).