Added PostUpdater DSO and spec.

This is a "permissive" DSO at this point, since entity-level validation
for modified Posts is presently broken (see Issue #89). Specs are
woefully incomplete (see the disabled/pending specs in
post_updater_spec.rb).

Also added commented-out code for `PostController#update` action, which
depends on this new DSO.

Finally, fixed a typo in the post-edit view template.

RSpec: 692 examples, 0 failures, 8 pending (but see Issue #87)
RuboCop: 153 files inspected, no offenses detected