Co-authored-by: Allan Otodi Opeto <otodi.allan1@gmail.com>

splits the business of fetching (and fetching more) in the fetchTree
method into its own module

then another new module repurposes code from useExerciseResources and
leverages the new useFetchTree module to fetch the contents and
annotate them as needed for navigation the content tree specifically for
use with quizzes

useFetchTree (at least tries) to straightforwardly provide the intial
fetch of content while keeping itself up-to-date whenever the user
fetches more

…" btn

- use kolibri.lib.logging in place of console.*
- remove unneeded console.logs
- add guidance re: private variables
- remove unused search-related business
- fix bookmarks route (no topic_id)
- remove unused pagename constants
- code cleanup thanks to @rtibbles suggestions
- improved comments
- indicated follow-up issues where commented-out TODO work remains