Added tests and wrote documentation for post endorsement feature

[joonhoswe]

I wrote tests for the new post endorsement feature and also wrote documentation in UserGuide.md to inform the users on what new features were added, how they're being tested, and where to find the tests.

test/flags.js
Overview: Contains numerous tests for the various types of flags available
My Additions: Added tests for endorsing a post, checking its status to verify it was endorsed, removing an endorsement, and verifying its able to be re-endorsed again.

UserGuide.md
Overview: Contains all the features we've added over the last couple of sprints, how we tested them, and where users can find those tests
My Additions: Added user documentation for the purposes mentioned above.

CoverAlls Result after adding new tests:

[coveralls]

Pull Request Test Coverage Report for Build 13557950926

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.639%

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Totals
Change from base Build 13557881240:	0.0%
Covered Lines:	22418
Relevant Lines:	25705

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💛 - Coveralls

[savannahxunc]

Looks great! Your tests and notes in the 'UserGuide.md' are very comprehensive.