Hide option to add new visibility for individual feed item

This can be confusing for manual content creators. Instead, visibilities
can be added through the taxonomy editor.
Remove TwentyTwelve from the repository
Ignore Twenty Twelve theme files
If an author is not available, use the author of the item
If a source is not available, use manual entry
Fill link_url for manual items as well
Merge pull request #57 from washingtonstateuniversity/fixes

[PR] Various updates and fixes from production use