Fixed `BlogHelper#alert_for` so its fixture quacks like Rails.

Rails' `ActionDispatch::Flash::FlashHash` isn't really a `Hash`; and it isn't really an `Array`; and it's simplistic in that, although it can accept one message for any number of arbitrary message "levels", it can only accept *one message at a time* for each. If our `#alerts_for` function (and its supporting class) was to work with The Real Rails Thing, we couldn't just pass in hashes or arrays of hashes. Oops. Fixed.

This let us make one critical change that was needed to build multiple flash message renderings correctly.

RSpec: 103 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 38 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1
Flash messages now display; HTML is not escaped on output.

RSpec: 103 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 38 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1
Various notes and fix-ups:

* FIXME note in `PostsController#create` re need to hook `outcome.errors` into the view (flash messages) again; and
* `CreateAndPublishBlogPost` spec now includes verification of case where neither `:body` nor `:image_url` is specified;
* `CreateAndPublishBlogPost` class has `#validate` method that TRIES to Do The Right Thing. Alas, `.required` is a busted filter;
* Finally getting around to fixing a copy-and-paste bug in `config/initializers/session_store.rb`.

Note that `Blog::Blog::AddingPosts#new_post` is clunky; should module be moved into, e.g., `::Blog::Support::AddingPosts`?

RSpec: 111 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 38 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1
Added `image_url` property to `Post`.

This had the expected ripple effects, including the `PostsController`, the view template for a new `Post`, and specs.

RSpec: 114 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 38 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1