Make discovery of communities (federated and local) first-class in user setup

[Klobenda]

For backend issues, use lemmy

Is your proposal related to a problem?

Discovering communities, even local ones, does cause a lot of user friction.

Displaying a first-time-user-setup page where instance-suggested communities can be subscribed to, and communities can be searched for (and subsequently subscribed to) would assist.

Describe the solution you'd like

Something like this brief mockup:

Makes the following somewhat necessary:

• Instance owners can define a list of communities they think their users would like and should be subscribed to on join.
• Communities can have tags, short descriptions, and their metadata could be federated (with easy-opt-out) to other instances to ease discovery.

Describe alternatives you've considered

The current existing behavior.

Additional context

Back-end prerequisite to some of this: federate known (and opted-in) community metadata https://github.com/LemmyNet/lemmy/issues/2951