SiteIcon PUT should go through validator to resize and get _cache-max-age

[cdent]

Lots of people PUT rather large SiteIcon tiddlers, which is expensive on a variety of dimensions. If we wanted, we could use a validator to resize it and set a _cache-max-age header.

We could also consider doing these adjustments client side but that would not catch all interfaces.

[cdent]

See my notes on wand

[cdent]

This seems increasingly relevant: If a large SiteIcon shows up on tapas the UI can be unusable for a bit while the images are loading. The slowness has made me unfollow people with large icons.

[pmario]

+1 for scaling down big icons.
Can you fix existing icons too?

The slowness has made me unfollow people with large icons.
IMO that points out, that there should be a mechanism to "inform" users about there issue. A "notifications" type of thing. see: #1106