Adding local site settings to reject federated upvotes or downvotes.

[dessalines]

• Should help defend against downvote spamming instances.
• Fixes #4086

Notes:

• I don't have any tests for this currently (there wasn't any for enable_downvotes either), and I'm not sure what the best way to do that is.

[dessalines]

Okay this is ready to be looked at again.

[poVoq]

Are these new settings exposed via the API so that 3rd party clients can easily check what vote options are available and display them accordingly?

See also: https://github.com/Xyphyn/photon/issues/459

[dessalines]

Yes they come back with GetSite.site_view.local_site