Remove presence/eyeballs/online status from the app

Remove presence/eyeballs/online status from the app. This includes backend/frontend and Redis stuff.

As discussed in https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8821#note_286519966

Because of the following reasons, we should remove the presence tracking code 🗡,

1. It's slowing down Redis
2. Presence/eyeballs/online status has been broken for a while. This may be the cause of why there are so many arguments for each Redis query (they're built up and stuck on a certain status)
3. We only show eyeballs in the 30 person roster in the right-toolbar but if there are more than 30 people we only ever show the first 30 people with no sorting so it's not that useful for big rooms
4. People don't like how we stop mobile notifications if you have a desktop client open, https://gitlab.com/gitlab-org/gitter/webapp/issues/1846

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cc @viktomas