Rework `Avatar` indicator to be more general-purpose (#4073)

This PR reworks the way we add indicators to `Avatar`s to make them more
general-purpose.

Previously we had logic specific to the availability indicator embedded
in the `Avatar` component, which made it unwieldy to repurpose for
something else.

Now the `indicator` is just a slot that we can put anything into.

Release Notes:

- N/A