Feeds show view does not automatically fetch feed entries, it just shows entries already in the database.

In the future feed fetching will happen in a background job that will be triggered:

- by a user clicking a "refresh" button
- automatically at set intervals (for instance every hour or so)
Sidebar menu in the feeds index view now have a light brey background color. It looks much better now.
Added a refresh button to the feeds index view. Still non-functional.
Fixed broken html in the feeds index view.
Fixed broken internationalization in the navbar.
Order feed entries by published date in the view.
Remove unnecessary href attribute from feed entry links.
Removed unnecessary href attribute from sidebar links in the feeds index view.
Feed entry background is a very light gray.

This better highlights the important parts of the page.
Fix broken html in the feeds show view.
Use html5 data-* attribute instead  of class attribute to highlight the selected feed in the sidebar.
Work in progress: add a Refresh button that loads new feed entries via Ajax.
The Ajax call launched when clicking on Refresh now downloads an HTML fragment instead of JSON data.

It's far easier to format HTML fragments on the server, with ERB and internationalization, sanitization etc helpers, than doing it with javascript on the client.
Fix broken unit tests.
Moved the code that fetches and parses feeds to a new class.
Reorganization of a unit test.
Feed title and url are retrieved from the feed XML at the same time as new entries.
Every time a feed is fetched, save in the database the etag and last-modified headers received in the response (if any).

In the future these values will be sent back the next time the feed is fetched, to help in caching indicating the server to respond only with new entries.
Minor change in a unit test.
When fetching a feed send if-none-match and if-modified-since HTTP headers, if able, to indicate the server than only new entries should be sent.