Added :post_status attribute to PostData class.

This is NOT an ORM field! it's purely a prop to allow us to toggle
public/private status of a post and have that difference reflected in
the state of other properties of the instance (specifically `pubdate`).

RSpec: 713 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected
PostCCO#from_entity now sets post_status properly.

Remember that `:post_status` is the synthetic attribute we added to the
`PostData` model in the previous commit. This commit updates the
`CCO::PostCCO` cross-layer conversion object to initialise the
attribute correctly based on the `pubdate` attribute of the entity
being converted. (There is presently no need for a status-type field at
the *entity* level; this is being added solely for the
implementation-level UI.)

RSpec: 715 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected
Added PostsHelper#status_select_options method.

This builds the options for a `select` helper (using the
`#options_for_select` form-options helper), using the `post_status`
attribute added previously, and defaulting to 'draft' if that attribute
is somehow not set. This will allow us to add the post-status UI to the
create-post form.

RSpec: 723 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected
PostData#post_status now defaults to 'draft'.

Instead of a simple `attr_accessor` for `post_status`, we have an
`attr_writer` and a custom reader method that sets the return value to
`public` if the `pubdate` field is set. We've also added a
"post_status" field to the `_post_edit_fields` partial. All's well by
the existing specs…

RSpec: 723 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected
New-post form "status" field defaults to "draft".

Which is the letter of Issue #100. So why aren't we closing it yet? We
*could*, with a straight face, but… read the whole sad story in the
issue comments.

RSpec: 724 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected
Merge pull request #103 from jdickey/issue-100-stage2

Stage 2 (of a currently-known 3) to properly close Issue #100.

RSpec: 724 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected