Before:

```
$ time trial tests/events/test_utils.py > /dev/null

real	0m2.277s
user	0m2.186s
sys	0m0.083s
```

After:
```
$ time trial tests/events/test_utils.py > /dev/null

real	0m0.566s
user	0m0.508s
sys	0m0.056s
```

without exceeding size limits.

Now that we allow admins to include events in prejoin room state with
arbitrary state keys, be a good Matrix citizen and ensure they don't
accidentally create an oversized event.

…prejoin-state

should have done this in #14668