Added "real" post-editing view.

Also

* refactored PostsHelper to replace previous #new_post_form_attributes
method with call to private method shared with new
#edit_post_form_attributes method;
* added https://github.com/fuksito/markdown-toolbar to edit-post view;
functional but clunky (see Issue #88).

Things that still need doing:

* get the update-post action added to the PostsController;
* add the toolbar to the new-post and new-/edit-user views as well.

RSpec: 657 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected