Create a preloaded feed for the test network

[mplorentz]

Currently if you create an account on the Planetary Test Network we load the preloaded feed with a bunch of content from the main network. This is confusing because you can follow pubs and users from the main network but you can never replicate new posts from them.

We should create a separate preloaded feed that has content from the test network. This will made the Discover and Your Network tabs, as well as default lists of pubs, behave more like production.