Add user input validation for existing features

[IsaacCheng9]

Summary

• User input validation is required for things such as creating expenses with unexpected field data types (strings in Amount, entries which are too long, etc.) and null inputs.
• We also need to add text wrapping or ellipsis for long usernames.
• Ensure that feedback is shown to the user for invalid inputs.

Examples of Unwanted Inputs

[IsaacCheng9]

This has been partially implemented in the frontend, but there may be some edge cases to check. Most of the remaining work in this is in the backend.