Adding the API as a submodule

Summary:

• Adding api.sh that runs the API if needed (Linux-only)
• When unit testing, run the script to check that the API is running (Linux-only)
• Displays a warning on Windows

Fixes T311

Test Plan:

• On Linux: run the tests without the API running, the gradle task should fail and tell you
• On Linux: run the tests with the API running, it should run the tests normally
• On Windows: run the tests, it should print a warning because it cannot check if the API is running
• On MacOS: currently it behaves as on Windows, because I do not know if MacOS handles this bash script or not

Reviewers: Info-Screen, WyldBot

Reviewed By: WyldBot

Maniphest Tasks: T311

Differential Revision: https://phabricator.wildfyre.net/D206