Server: Improve mock server for CLI/frontend development

[camerondurham]

Create mock server for API endpoints described in design doc: Runner Project Details

This functionality currently exists. However it would be nice to have a separate mock server
for specifically testing frontend/CLI and so changes can be made on server without interfering with other development.

Acceptance Criteria:

• Mock GET /api/v1/languages (added in Add project skeleton and basic structure #11, can copy into mock server)
• Mock POST /api/v1/run (added in Add project skeleton and basic structure #11, can copy into mock server)
• Mock POST /api/v1/run with random response (stretch)
  • Return mock "syntax error" like response
  • Return mock "runtime error" like response
  • Return mock successful response (prints out some message to stdout/stderr, no errors)
• update Makefile commands to support make run-mock-api and run the corresponding mock server

[camerondurham]

This is currently added in #11

Some improvements could help front-end development, edited AC.

[camerondurham]

Closing since this is no longer a priority.