Add the ability to add a description field to a scope

[ry]

On the scope page it should render the description in markdown.

It might also be nice to have it for user profiles.

[jollytoad]

Would be nice to have the option for an entire 'Overview' tab for a scope, which renders a full markdown document. I think this would be especially useful for @std to provide some general information about the standard library.

[iuioiua]

Having a small text description field and providing a link for further information might also be fine. See #507.