Decoupled BaseSummariser from BlogSummariser.

Next up: taking the BlogSummariser code (used by BlogController#index)
and creating a workalike PostsSummariser (for PostsController#index)
before killing the Blog stuff.

RSpec: 854 examples, 0 failures (but see Issue #87)
RuboCop: 163 files inspected, no offenses detected