Fix: addMember function to detect role automatically

[shpomp]

Description

addMember function in users.controller is changed to automatically detect the member role according to the parameters that are passed to the POST endpoint.

Fixes #247

How to test?

npm run dev and test in swagger docs. http://localhost:5000/api/documentation/#/Members/post_users__userId__boards__boardId__members__memberId_

Checklist

• [x] I have performed a self-review of my own code
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• [x] I have commented on my code, particularly in hard-to-understand areas, if your code was simple enough mark the box anyway
• [x] I have made corresponding changes to the documentation, if your code was simple enough mark the box anyway
• [x] This PR is ready to be merged and not breaking any other functionality