Refactor scoreboard usage

In preparation for team colors, we need to unify scoreboard usage into a
single scoreboard per player.

Rather than creating a new scoreboard for each desired sidebar
objective, we provide an interface FSidebarProvider that allows
implementation-agnostic retrieval of the title and lines of the desired
sidebar, along with appropriate methods in the new implementation of
FScoreboard.

The new FScoreboard implementation manages the currently-visible
sidebar, with methods to set default (non-timed, with update interval)
and temporary (timed, no update interval) sidebars.

Temporary sidebars are replaced with the default sidebar upon
expiration.