Bug: Replacing posts hosted on cdn.donmai.us didn't work.
Cause: Original files on cdn.donmai.us are hosted under /var/www/danbooru/original/, but replacements
were trying to store them directly under /var/www/danbooru, which failed with a permission error.
We were trying to store them in the wrong directory because we didn't respect the `original_subdir`
option when generating file paths.
Add option to allow for storing original files under an `original/`
subdir. Previously preview, crop, and sample files were stored under
subdirs but original files were stored in the top-level directory. This
is optional for backwards compatibility.
Move Post#humanized_essential_tag_string to TagSetPresenter#humanized_essential_tag_string.
This allows humanized_essential_tag_string to reuse the same set of tags
already fetched by the tag set presenter for the sidebar.
This avoids fetching the tag categories from memcache again (via
Post#typed_tags) when we're already fetched the tags once before.
This also means it's no longer necessary to cache humanized_essential_tag_string
itself in memcache, since it can be generated as quickly as the sidebar taglist.
