Take default og:site_name from sphinx `project` config value

[rkdarst]

• The standard project Sphinx config value seems like a reasonable default value for the site_name.
• This uses that as a default, if ogp_site_name is not set.
• This will change the default, and add og:site_name for many projects where it previously wasn't set.

Perhaps I should have asked before doing this, since the last point may be a blocker. But it was easier to do than think too much...

[rkdarst]

The reason I think this is useful is that I can add this extension to any site and it's closer to being functional even without any configuration.

Would we want a way to tell it to disable the og:site_name property? Right now, site_name can't be unset (assuming most sphinx projects have project set).

[TheTripleV]

Yes, there should be a way to turn this off, similar to ogp_image_alt.

Default ogp_site_name to None.

if ogp_site_name is None:
  use project
elif ogp_site_name == False:
  omit og:site_name
else:
  use config site_name

[rkdarst]

Updated and works in my local test.