Trending backend algorithm and files

[anerishah18]

I've typed out the algorithm and made the necessary files for this feature. The algorithm is contained in FindTrending.java and the rest of the files are there for frontend/backend support.

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[anerishah18]

@googlebot I signed it!

[anerishah18]

I renamed the file and will add the tests in the next push because I'm waiting on some file structure setup from Toni.

[anerishah18]

I see that in my branch on github, both FindTrending.java and FindTrendingCharities.java are listed as files, but on my local editor, I rename the former to the latter, so I'm not sure how to remove that FindTrending file from github.