Extracted and tested the database interactions from Category

[psh]

The goal of this PR is to simplify Category and get decent tests around its database interactions, particularly the database migrations from version-to-version.

To that end

• Database interactions were extracted into a CategoryDao
• Tests were added
• Classes that formerly called save(), find() and recentCategories() on Category were updated to use a data access object

[codecov-io]

Codecov Report

Merging #1023 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #1023      +/-   ##
=========================================
- Coverage     3.7%    3.7%   -0.01%     
=========================================
  Files         105     106       +1     
  Lines        5160    5162       +2     
  Branches      485     485              
=========================================
  Hits          191     191              
- Misses       4956    4958       +2     
  Partials       13      13

Impacted Files	Coverage Δ
.../nrw/commons/category/CategoryContentProvider.java	13.79% <ø> (ø)	⬆️
...e/nrw/commons/category/CategorizationFragment.java	0% <0%> (ø)	⬆️
...c/main/java/fr/free/nrw/commons/data/Category.java	0% <0%> (ø)	⬆️
...ain/java/fr/free/nrw/commons/data/CategoryDao.java	0% <0%> (ø)
...n/java/fr/free/nrw/commons/CommonsApplication.java	27.65% <0%> (ø)	⬆️
...in/java/fr/free/nrw/commons/data/DBOpenHelper.java	20% <0%> (ø)	⬆️

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[neslihanturan]

Hi @psh , can you please solve the conflicts, so that I can merge them. Thanks!

[neslihanturan]

Thanks @psh !