Remove rounding from PostsItem
Remove rounding from PostsItem

PostsItem became rounded due to a change of defaults in the new version
of material-UI. It's very mildly ugly, so don't do that.

Discordius/Telescope,Discordius/Telescope,Discordius/Lesswrong2,Discordius/Lesswrong2,Discordius/Lesswrong2,Discordius/Telescope,Discordius/Telescope,Discordius/Lesswrong2