Add support for widget image

[Puncher1]

Summary

This PR adds a new method widget_image_url to Guild, which returns an URL to the widget image.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

I'm still unsure what the proper name for this is, since it's inconsistent with other methods that are similar, i.e. Asset but we talked about this in the discord server.

[Puncher1]

Yes. I couldn't find a good name for it. I first thought about widget_image_link but idk if this is the way to go.

[Puncher1]

How about turning this into an helper function in utils.py like oauth_url?