Add Category Enum and Refactor

[jq1836]

Resolves #145, #159

Overview

To allow us to have finite categories for matching, we make use of enums.

Complexity changes

• easy -> Easy
• medium -> Medium
• Hard -> Hard

Category changes

No longer free text, but string enum instead, the following are current possible values:

• Algorithms
• Arrays
• Bit Manipulation
• Brainteaser
• Databases
• Data Structures
• Recursion
• Strings

Other changes

On top of the above, major refactoring of frontend and question-service was done to improve code quality.

[jq1836]

Only merge after DB is prepared, i.e. all questions purged or updated to new format.

[SelwynAng]

LGTM overall, aside from some comments mainly for the question-service! To merge only after removing all existing questions in the database, since current question format will leads to errors on the frontend.