Customavatar: refactor custom avatars and update style

[LittleKitsune]

• use a Promise to remove an avatar
• check if the user has an avatar before setting it - fixes problems of choosing which avatar which is selected in case there is a gif, and a png.
• use arrow functions instead of the .bind(this) hack - better efficiency
• directly set the avatar to the target user's user object, so less refreshing is needed if the targeted user did not have a custom avatar previously

[LittleKitsune]

yeah lol idc

[NeonLightPWO]

ok then