Update updateUser

[vaasuu]

• change POST to PATCH. More semantic; you know it allows partial updates
• get user id from url params; consistent with other endpoints
• better validation and edge case handling

[coveralls]

Pull Request Test Coverage Report for Build 6922586586

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 23 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 90.967%

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Files with Coverage Reduction	New Missed Lines	%
backend/coverage/backend/controllers/users.js	23	89.97%
<!--	Total:	23		-->

Totals
Change from base Build 6922031762:	-0.2%
Covered Lines:	618
Relevant Lines:	681

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💛 - Coveralls