Room and group names should be limited to a sensible length

[matrixbot]

This issue has been migrated from #8151.

---

Description

You can set room names and topics to absurdly long strings, which don't fit in a matrix state event. As a result, when you create a room in Element doing so, you get a room with no name/topic as the state event failed to send.

Synapse should fail the /createRoom call instead immediately instead of creating a room with no name/topic.

There's a similar issue with groups, though while the group is created successfully, being able to create groups with unlimited-length names is not ideal.

We already limit the size of usernames to 512 characters, returning a 400 M_UNKNOWN. We can do this again for now, but it'd be nice to get an appropriate error code to return so that clients can show a helpful reason as to why creating the room failed. Related spec issue: https://github.com/matrix-org/matrix-doc/issues/1646

Steps to reproduce

• Create a room with a reaaaaaaally long name (>65K chars)
• The /createRoom call fails with a 500
• Notice that a room is created anyways

Version information

v1.19.0