Opening content from the search provider is broken with kolibri_explore_plugin

[dylanmccall]

With the change proposed in #37, the Endless Key app fails to display content when that content is opened directly, such as through the GNOME search provider. This can be tested by running kolibri-gnome with a kolibri: URL specifying some piece of content:

kolibri-gnome kolibri:c/8d04cf37798357a48dea25c6b7d6cbec

The app is expected to show that content. Instead, it shows the Discovery page. This is because the Explore plugin expects a different URL scheme for individual pieces of content than in the Learn plugin. The app is trying to load a URL like...

https://127.0.0.1:8080/explore/#/topics/c/8d04cf37798357a48dea25c6b7d6cbec

But the Explore plugin expects...

https://127.0.0.1:8080/explore/#/topics/000409f81dbe5d1ba67101cb9fed4530/c/8d04cf37798357a48dea25c6b7d6cbec

Ideally, it would be better to improve compatibility in the Explore plugin itself here - get it to figure out the channel ID for the given content node and redirect accordingly - to avoid needing to fetch information about content nodes in the desktop app.