Spectated communities are never closed

[ilmotta]

Problem

When a user opens a community but never joins it, the community stays in the spectated state and accessible in the Opened tab in the home screen, forever, even after logout. Each spectated community will eat considerable resources, much like a community the user has joined. This is a form of resource leak.

The main performance cost here for the mobile app comes from the overhead of keeping the state in sync as signals arrive for a community (esp. a large one), given that we globally process signals independently of the current screen the user is at.

Original discussion in #20508 (comment).

Implementation

Based on a private discussion with @cammellos:

1. Spectated communities would be sort of garbage collected every X time (like once every 7 days if the user hasn't interacted with the spectated community).
2. Limit the insertion of new spectated communities to a max number of 5-10, or any small(ish) number and in a FIFO fashion.

This automatic approach should be preferred because it could be confusing to users to leave/close/unspectate communities manually. Additionally, it wouldn't be obvious to users why manually closing spectated communities is necessary.

Scope

Although the problem exists in theory, it shouldn't affect users at the moment because creating communities is disabled on status-desktop and creation is also not available on mobile, therefore we should postpone work on this issue.