Add fallback to group avatar in case thumbnail not found on server

[agent3bood]

No description provided.

[agent3bood]

Fix https://github.com/vector-im/riot-web/issues/3563

[turt2live]

I would almost expect this to be fixed on the server side to be honest. The thumbnailer seems broken, but if it can't be fixed then this seems like a relatively okay solution to me. (Although I have no authority on this aspect and I haven't tested it myself).

[agent3bood]

Agree, fixing it on tbe server is the way to go, but this fix is adding a fallback incase thumbnail not found on the server.

[agent3bood]

Also this issue
element-hq/element-web#2581

[lukebarnard1]

So this potentially allows for an SVG to show be shown as the room avatar? We shouldn't be showing any SVGs afaik. Better for this to be done on the server I think.

[ara4n]

yeah, i see the intention here, but the fact that it displays the original SVG if a thumbnail is missing (resolving element-hq/element-web#2581) is not really ideal - as Luke says, we go to pains to not currently show any user-generated SVGs, as it is trivial to sculpt malicious SVGs (e.g. billion lol attacks). If anyone is wrangling thumbnailing of SVGs it should be the sender client (or possibly the server)'s responsibility.

Sorry for not being able to merge :(

[agent3bood]

Thanks all for your comments, so this issue is a backend problem not the client.