changed fetch method to add candidates

[NatsGt]

Description

Replaced postData fetch method with global fetchFromDb

Fixes #282

How to test?

Add candidates on a board

Checklist

• [x] I have performed a self-review of my own code
• [x] I have followed the name conventions for CSS Classnames and filenames, Components names and filenames, Style filenames, if you are in doubt check the the project README.MD and here https://github.com/HackYourFuture-CPH/curriculum/blob/master/review/review-checklist.md
• [x] I have commented my code, particularly in hard-to-understand areas, if you code was simple enough mark the box anyway
• [x] I have made corresponding changes to the documentation, if you code was simple enough mark the box anyway
• [x] This PR is ready to be merged and not breaking any other functionality

[senner007]

We're using async almost everywhere now. I think it'd be better to standardize on that and remove the Promise resolution here. Also, any error will be caught at the error boundary anyway, right? Or do you want to catch it here?

Please note that using async/await syntax also returns Promise objects. The good thing about using async/await is that it is ubiquitous across languages even though the implementation underneath differs slightly.

[NatsGt]

Made a change to return data from fetch method since the response.json throws an error if the method is not get.