Added :edit action to PostsController.

Added :edit action (only) to routing, and have a complete dummy
edit-post view template. All will come.

RSpec: 681 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
Killed obsolete Gem `blind`.

We've just reminded ourselves that it doesn't work, and we've found a
workaround that does: touching the nonexistent view-template file
(creating a new file of zero length). Specs work. :grimace:

RSpec: 681 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
Second whack at PostsController#edit.

Now does not assign variable unless authorisation succeeds. Specs cover
all three authorisation scenarios (author, guest, non-author user).

Routing updated to allow all actions for posts *except* :destroy (we
never delete posts) and :index (the Blog#index method/action does this,
at least now).

RSpec: 685 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
Broke partials out of post/show template.

Working our way back to the post/edit template. Honest.

RSpec: 685 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
Harmonised Posts/new and Posts/edit templates

By adding a partial for the field controls that are the bulk of the
form.

RSpec: 685 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
PostsController#update action method added.

RSpec: 695 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
More (temporary) view cleanup.

"Temporary" because, in a world with adequate time, many/most of these
small partials would be converted to decorator methods, as they're
easier to test. But oh well.

RSpec: 695 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
[Fix #90] RSpec 3.x at last!

Currently 3.1.2, but the important thing is: goodbye, 2.99! Another
unwanted boat anchor cut from its chain and thrown overboard.

RSpec: 695 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
New feature spec for edit-post button for author.

RSpec: 698 examples, 0 failures (but see Issue #87)
RuboCop: 149 files inspected, no offenses detected
Users other than author cannot edit existing post.

The Guest User and any registered user other than the post author are
prevented from editing a published post, either by directly navigating
to the "edit" page (which has been blocked since Pundit edit-action
policies were implemented), or by clicking an "Edit <article-name>"
button (which is only shown to the author).

RSpec: 702 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected
Merge pull request #98 from jdickey/issue52-stage2

Adding the controller action, UI and associated code for editing posts.

RSpec: 702 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected