This commit fixes some superficial instances of Broken Access Control
(https://owasp.org/www-project-top-ten/OWASP_Top_Ten_2017/Top_10-2017_A5-Broken_Access_Control).
There may be some more instances of this, as authorization was only done
after most of the controllers were done (big mistake).
Some refactoring was also performed, where Route Model Binding with DI
(dependency injection) was used whenever possible, to increase
testability of the codebase.
Some reused code was also moved to Helper classes as to enforce DRY;
There may be some lines of code that are still copy-pasted from other
parts of the codebase for reuse.
Non-breaking refactoring changes were made, but the app as a whole still
needs full manual testing, and customised responses to HTTP 500
responses. Some errors are also not handled gracefully and this wasn't
checked in this commit.
