Dendrite does not validate `senders` and `rooms` IDs in filters

*This issue was originally created by [**@ShadowJonathan**](https://github.com/ShadowJonathan) at .*

### Background information

- **Dendrite version or git SHA**: 83b9cb4d79123ec89e18a46d5202e74a9e9305b8
- **Running in Docker?**: yes

This was found while running Complement, so build information from [this Dockerfile](https://github.com/matrix-org/complement/blob/277e25800c9b51f5dc0fb5d89430cf99d817267d/dockerfiles/Dendrite.Dockerfile) also applies.

### Description

When working on [matrix-org/complement@`8021d43` (#259)](https://github.com/matrix-org/complement/pull/259/commits/8021d43102845e7dcb71e2d58455034c33131393), dendrite failed the tests with the following logs;

```
client.go:392: POST hs1/_matrix/client/r0/user/@alice:hs1/filter => 200 OK (84.506966ms)
invalid_test.go:180: Expected 4XX status code, got 200 for testing filter map[room:map[timeline:map[rooms:[not_a_room_id]]]]
client.go:392: POST hs1/_matrix/client/r0/user/@alice:hs1/filter => 200 OK (57.808076ms)
invalid_test.go:180: Expected 4XX status code, got 200 for testing filter map[room:map[timeline:map[senders:[not_a_sender_id]]]]
```

This test comes from sytest, and i am pretty sure that dendrite should at least try to validate these strings to valid User IDs and Room IDs

Start at Dendrite's POST /_matrix/client/r0/user/{userId}/filter endpoint and review the invalid filter cases from Complement or sytest described here. The work is done when invalid senders and rooms IDs are rejected with a 4XX response instead of accepting the filter.

Written by the indexing model from the issue text.