Added 404 page writer and test for it.
Updated 404.html
Revert "Added 404 page writer and test for it."

This reverts commit 1106be9b230046aa3f89b1d6ecf86b9dab49d64a.
Updated 404.html with default content.
Implemented tags.json writer.
Written test for tags.json writer.
Added error message for when fetching tags.json fails.

Also, updated default content block's id and fixed indentation.
Added TagJSONWriter to the default setting files.
Merge branch 'tag_pages'