Refactor: replace Flask-Restful with own, simple implementation

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

• Flask-Restful is not really maintained right now and hard-pins us to specific Flask versions
• before_request and after_request callbacks meant for API blueprint are also applied to static blueprint

What is the new behaviour?

• We use our own MethodView-based wrapper
• Bumped apispec, marshmallow and Flask