Add avatar plugin.

[goto-bus-stop]

Adds a uw.avatars API. The public API is:

• uw.avatars.getAvailableAvatars(user) - List available avatar descriptors.
• uw.avatars.setAvatar(user, descriptor) - Set the user's avatar to one of the getAvailableAvatars() descriptors
• uw.avatars.supportsCustomAvatars() - Check if custom avatar uploads are possible (requires configuring a file storage path)
• uw.avatars.setCustomAvatar(user, stream) - Store the avatar from the readable stream and set it as the user's avatar

The default implementation has these features:

• "Magic" avatars, like the default 'sigil'. These are generated based on user data (or randomly). A plugin could use this to add gravatar support for example
• "Social" avatars, which uses the profile picture URLs of each social auth method that a user has used

A client is expected to show a list of available avatars to choose from, and an image upload form if uploads are enabled.

TODOs

• Probably add a "gallery" type to the default implementation so we can use yoongi and panda's solid af designs
• Add size and dimension restrictions to uploaded avatars—dynamically resize and compress
• Have an assets plugin of some sort to store and serve the uploaded avatars Asset store for uploaded files #397

After this is done, the pending social account stuff should use the avatar selection screen on the client end. That way we don't show user's faces in chat immediately if they signed up with their google account for example. (This is why google signup is not enabled on wlk.yt fwiw 🤣 )