Refactor asynctasks to Rxjava implementation

[vanshikaarora]

Description (required)

Refactored asynctasks to Rxjava implementation

Fixes ##2698 Refactor Improve Peer Review

Changes made

• Deleted classes SendThankTask.java and CheckCategoryTask.java
• Added RxJava elements instead

[codecov-io]

Codecov Report

Merging #2707 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #2707      +/-   ##
=========================================
+ Coverage    2.71%   2.71%   +<.01%     
=========================================
  Files         268     266       -2     
  Lines       12864   12848      -16     
  Branches     1147    1147              
=========================================
  Hits          349     349              
+ Misses      12489   12473      -16     
  Partials       26      26

Impacted Files	Coverage Δ
.../fr/free/nrw/commons/di/ActivityBuilderModule.java	0% <ø> (ø)	⬆️
...a/fr/free/nrw/commons/review/ReviewController.java	0% <0%> (ø)	⬆️

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[vanshikaarora]

Thanks for approving the PR @maskaravivek :)