Matrix >= v1.0.0 limits display name field to 100 chars

[ulope]

Sub issue of: #3387

Problem Definition

synapse >= v1.0.0 introduced (matrix-org/synapse@d16c637) a length limit on the User displayname field of 100 characters.

This breaks our use of this field as a way to publish hex encoded ecrecover signatures since those need 132 characters.

Possible solutions

1. Get Matrix to change the limit: New 100 char display name length limit is too restrictive matrix-org/synapse#5870
2. Switch to base64 encoding (which would fit at 89 chars for a signature)
3. Switch to use avatar_url field which has a limit of 1000 chars

All but 1. would be backwards incompatible (which currently should be doable) and would need coordination with the light client (ping @andrevmatos).

[ulope]

Good news, the Matrix team increased the length limit to 256 chars: matrix-org/synapse#5906!

Now we only need to wait for them to make a new release. I asked in the linked issue if there's an ETA for that.