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.
88a177e1d5