Show time remaining for other players' turns in multiplayer and some other features

[StenAL]

This PR contains the following fixes/features:

• The remaining time for another player to shoot is now shown in multiplayer games.
• Single player Championship games now work, previously they started the track fetching code was not implemented.
• Username validation now fires after the text field is updated. This means single-character usernames can be used, previously they couldn't because they were failing validation.
• Client username can be set through a CLI flag to make testing faster and easier
• Client CLI argument parsing can now be tested better because it does not call System.exit anymore
• Building the server with no changes is a few seconds faster. Previously all sources were recompiled every time because the Database class was fully commented out.

This also includes some deobfuscation work in preparation of making the Try Again Now button work when a client is disconnected from the server.

[PhilippvK]

@StenAL Could you please check the failing unit test?

[StenAL]

I fixed the test now, I had accidentally written assertNotEquals instead of assertEquals in one of my new tests. I squashed the fix into the Add CLI flag to set username commit.