Newsroom Resources fix data-limit attribute, extract data fetching code, and create template for redesign

Resolves #364 (closed)

Resolves #361 (closed)

• Fixes data-limit attribute for the "image" template
• Introduces "cover" template, similar to "image" but styled
• Extract data fetching from widget into /api/eclipsefdn.newsroom.js
• Adds a few unit tests for widget and data fetching code
• Make widget code more consistent with other widgets
• Creates documentation pages for each template