Un-ignoring a user does not show the messages which was sent while ignoring the user

### Steps to reproduce

1. Alice and Bob are in a room, and some messages are there.
2. Alice ignores Bob. Bob's messages vanish
3. Bob send new messages. Alice do not see them
4. Alice un-ignores Bob. Old messages from Bob are now visible again, but new messages are not visible

I guess there is no other solution than to perform an initial sync when un-ignoring a user, since the client do not receive those event.

Note: could be nice to create a label `A-Ignored users` as per Element Android project https://github.com/vector-im/element-android/labels/A-Ignored%20users

### Outcome

#### What did you expect?

See all the messages sent by the un-ignored user

#### What happened instead?

New messages are not visible

### Operating system

Any

### Browser information

Any

### URL for webapp

develop.element.io

### Application version

latest

### Homeserver

matrix.org

### Will you send logs?

No