Added `Blog::BlogHelper#alerts_for` method.

…after futzing around with trying to make it a proof-of-understanding for Draper decorators, and then remembering that decorators decorate models, but the #alerts_for method is called from the view. Oops. Moving it over without touching any of the unit-tested actual implementation was an even *better* proof of understanding.

In other news, obviously, added Draper for (near-future) model decorators; see [here](https://github.com/drapergem/draper) for the doc on *that*.

RSpec: 101 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 34 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1
Source lines not covered by specs: 1