Fix ESLint errors and warnings

Fix ESLint errors:

• Reduce complexity in large functions
• Always use const instead of let when never reassigned
• Remove empty constructors
• Remove unnecessary escape characters from RegEx
• Add comment to empty functions of abstract classes
• Always return in mock test functions
• Add resolveJsonModule to compiler options in tsconfig.json to enable import from JSON files
• Remove unnecessary provider for MatDialogRef from app.module.ts

Fix ESLint warnings: Remove unused imports, variables and parameters.

Add ESLint ignore pattern for unused variables: Since at some points we need to have unused variables or parameters, add ignore pattern which ignores variables which begins with an underscore.

NOTE: Since this MR contains refactoring of complex logic, this should be well tested.