-Added simple notification channel for Android O.
-Fixes notification building failure for background and popup player on Android O.
-Reduce notification channel importance to low to avoid making noise on every notification update.
- Migrate to fragments
- Fix#487
- Don't show "Open in popup mode" to channel links
- New backstack of videos
- Change the subscribers count to format using `NumberFormat`, for example some locales use `.` others `,`, this handles it automatically (and the old method had a bug for leading zero, e.g. 4.82.125 instead of 4.082.125)
- Add string 'subscribers' for channels with more than 1 subscriber (plural)
- Popup player chooses the default format and resolution based on the new preference (format)
- Fix taskaffinity of the router activities
- Show title before loading, as it is available from the items already loaded
* Use custom notification builder which has methods to set the artist
and title and also a method to set the playing state
* Update builder instead of view -> resovles deprecated warnings