Refactor: Category model

[ben-mathu]

Part of an ongoing code refactoring to move all models to a common models package.

Fixes #4782

Moved Category model to data.models package.

Tested debug on Pixel 4 (Emulator) with API level 30.

[codecov[bot]]

Codecov Report

Merging #4853 (ec4982b) into master (87b9648) will decrease coverage by 0.13%. The diff coverage is 0.00%.

:exclamation: Current head ec4982b differs from pull request most recent head 002d619. Consider uploading reports for the commit 002d619 to get more accurate results

@@             Coverage Diff              @@
##             master    #4853      +/-   ##
============================================
- Coverage     50.66%   50.53%   -0.14%     
+ Complexity     2231     2217      -14     
============================================
  Files           338      337       -1     
  Lines         15531    15508      -23     
  Branches       1358     1358              
============================================
- Hits           7869     7837      -32     
- Misses         7079     7089      +10     
+ Partials        583      582       -1     

Impacted Files	Coverage Δ
...va/fr/free/nrw/commons/category/CategoriesModel.kt	52.30% <0.00%> (-7.02%)	:arrow_down:
...java/fr/free/nrw/commons/category/CategoryDao.java	95.89% <ø> (ø)
.../nrw/commons/category/CategoryContentProvider.java	12.50% <0.00%> (-14.29%)	:arrow_down:
...w/commons/upload/categories/BaseDelegateAdapter.kt	35.29% <0.00%> (-11.77%)	:arrow_down:
...a/fr/free/nrw/commons/review/ReviewController.java	87.20% <0.00%> (-4.66%)	:arrow_down:
...ns/upload/categories/UploadCategoriesFragment.java	86.95% <0.00%> (-1.45%)	:arrow_down:
...java/fr/free/nrw/commons/data/models/Category.java
.../java/fr/free/nrw/commons/review/ReviewHelper.java	88.88% <0.00%> (+5.55%)	:arrow_up:

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[neslihanturan]

Hi, is this PR sleeping @ben-mathu ?:)