Use LiteUser in lots of places in the frontend

[aapeliv]

The frontend currently loads the user's whole profile in most cases, even if all it needs is the name + avatar. We discussed this at a few dev meetings, and for now I've implemented a GetLiteUser RPC that returns just the minimal info needed in the majority of cases in the backend. The main places where this does NOT work are the profile itself, and the map (that's a separate exercise).

In addition to the lite user RPC, there is also the GetLiteUsers RPC that gets many users: this allows fetching up to 200 users at a time in one backend call, which should significantly reduce the number of calls required e.g. when opening the map page.

This issue requires:

1. Creating a parallel useLiteUser(s) hook similat to the existing useUser(s) hook, and
2. Updating many uses of the latter hook to the new former hook.

couchers/app/proto/api.proto

Lines 22 to 28 in 711efe7

	rpc GetLiteUser(GetLiteUserReq) returns (LiteUser) {
	// Get minimal info about a particular user
	}
	rpc GetLiteUsers(GetLiteUsersReq) returns (GetLiteUsersRes) {
	// Get minimal info about a bunch of users
	}

[aapeliv]

@nabramow : the getLiteUsers API call is in this ticket.

Related to #4951