SF-3807 Store drafts in draft tab for speedy chapter navigation

[RaymondLuong3]

This PR removes the getDraft method from draft handling service in favour of the getBookDraft. It also implements a cache so that getting drafts is speedier when accessing a book that is already cached. The draft tab component has been updated to use the getBookDraft method which allows navigating between chapters to require minimal load time.

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This change is 

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.91%. Comparing base (12f0d90) to head (57368bd).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...late/editor/editor-draft/editor-draft.component.ts	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3899   +/-   ##
=======================================
  Coverage   80.90%   80.91%           
=======================================
  Files         630      630           
  Lines       40677    40694   +17     
  Branches     6578     6605   +27     
=======================================
+ Hits        32909    32926   +17     
+ Misses       6741     6740    -1     
- Partials     1027     1028    +1     

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