Changes from `Quart-Discord`:
- Switch to Discord API `v10`.
- Add `quartcord.models.User.display_name` attribute to show the global display name of the user.
- Add the `size` parameter to `quartcord.models.User.avatar_url` and `quartcord.models.User.default_avatar_url` to specify the resolution of the image (defaults to `1024`).
- Rewrite `quartcord.models.User.default_avatar_url` to match the [`index` calculation as per the new username system](https://discord.com/developers/docs/reference#image-formatting-cdn-endpoints).
- Add `quartcord.models.User.get_guilds()` method to return a list of the user's guilds from the cache if found, else make an API call to do so.
- Change the user `quartcord.models.User` model such that `str(user-object)` returns the `"username"` of the user, instead of the deprecated `"usernamediscriminator"` string.