Minor internal updates

* Improved notification images
* Fixes layout of options page
* Scrolls to selected feed when opening popup
* When using sidebar, popup will offer available subscriptions

Bugfixes and internal updates
* Failing feeds ignore cache headers
* Error tracking will no longer pick up errors from content
* Styling fixes

* Fixes initialization race condition
* Feeds with feed URLs are now linked

Version 0.4.0 brings significant performance improvements and bug fixes

* Fixes initialization race conditions
* Improved cache handling
* Increased fetching concurrency
* Whimsical illustrations because I found my graphics tablet
* Improvements to feed title handling

Minor updates to improve the getting started UI and error handling.

Feeds are now available via a popup in the toolbar.

To switch modes, change the view setting via the add on's options.