Hide from user

So when using chat sometimes a particular user can start harassing or creeping that user or sometimes its just a history that creates conflict.

Lets implement a feature that is like forcing someone else to ignore you. but the control is competely in the control of the taking the action.

I would limit the action to taking affect the next time the target user is logged in to avoid allowing a user to harass another user by causing their messages to blink off and on by repeatedly setting and then unselecting the hide from user feature.

The issue names no files, tests, or entry points; begin by reviewing the Matrix specification and existing behavior for user visibility and ignoring. Clarify the protocol scope, activation timing, and toggle semantics before implementation. Done means the behavior is specified clearly enough for interoperable implementation and testing.

Written by the indexing model from the issue text.