Improve the browser extension

[Jenselme]

To improve the raw result made in #156

Ideas

• Provide better error messages if the extension is used with its default configuration.
• Can delete an article after confirmation directly from the extension.
• Can unsubscribe from a feed after confirmation directly from the extension.
• Can save article and subscribe to feeds without closing the extension.
• Setup for translation.
• Auto save form updates (and display a saved status)? It’s not that great to have to update after editing tags.
• Display whether we are subscribed to a feed when opening the extension.
• Display whether we already saved the article when opening the extension.