Add guest user suffix, depending on organization setting

From feature level 216 (Zulip 8.0), realm_enable_guest_user_indicator is present in the response from register (and in events), which should control whether we add a (guest) suffix to users with that role.

The web app UI needs checking for which locations this appears, which likely needs a development environment in order to explore behavior for when this setting is enabled or otherwise.

We should also confirm that

• the suffix wasn't present unconditionally prior to this
• this behaves correctly in a backwards-compatible way

However, this will also need tests, which may need to be feature level (ZFL) dependent.