Feat/backend improvement

[limanjun99]

This PR upgrades the executor service so that it doesn't crash as much (e.g. when handling code that cannot be serialised to JSON). Tested with dev environment (https://feat-backend-improvement.d1fr5et3wgts3j.amplifyapp.com/).

Done:

• Added black as python code formatter

• Executor can read stdin and write to stdout

• All data should be serialisable now, either because they were originally serialisable (e.g. int, float, bool, str), by custom handling (e.g. dict, list, set, tuple), or simply by returning the object's str() value.

• Upgrade frontend to take in new response format

• Simple testcases for executor service

• Added error handling for syntax errors and runtime errors

• Update field "ok" to "executed" (response format from executor), to show whether the code was executed (either to completion or runtime error) or not (e.g. syntax errors, parameter errors etc.)

• Document the behaviour of the executor service so that we don't have to read the testcases to figure it out

• Add limits to JSON data sent back (to prevent executor lambda sending huge responses and using up all network outgress)

Future Work:

• Add stdin and stdout to frontend

[limanjun99]

Overall LGTM. Some concerns:

1. As shown in the image below the value of b is not displayed even when the execution was finished (Please look into this bug before approval)
2. The wrong execution message is displayed when there are too many executions due to time limit exceeded (Please raise an issue for this)

The cause for issue 1 has been identified, raised an issue for it (and a related issue as well).

Issue 2 will be fixed in a future PR.