Re-implement the "Summarise Content" Use Case.

[jdickey]

This is an analogue to Issue #68 and, more directly, to Issues #69, #70 and #71. Whereas Issue #68 concerned a reimplementation of the Publish a New Article Use Case, this issue addresses the Summarise Content Use Case, which was the very first Use Case implemented. As with Issues #69, #70, and #71, no specification for the reimplementation yet exists.

To close this issue, we need:

• a (new, never-before-done) spec for the Summarise Content Use Case analogous to that done for Publish a New Article; and
• a replacement for the existing Summarise Content Use Case.

[jdickey]

The specification for this Use Case now exists as a Wiki page.

Next up: a revision Gem implementing our current use-case protocol as specified, replacing the existing Use Case code.