Implement OAuth2 authentication with aiohttp to prevent blocking

[Askaholic]

Closes #409
Closes #404
Closes #406
Closes #402

Tested this on the test server and it appears to work correctly. The server continues to print debug info while the API request is happening. Would like to test it with more people though.

[Rackover]

DEBUG    Jun 12  09:36:06 LobbyConnection                Login from: 53471, axel12, 3013059539
INFO     Jun 12  09:36:06 aiomysql                       UPDATE login SET ip = '68.98.11.83', user_agent = 'downlords-faf-client', last_login = NOW() WHERE id = 53471
INFO     Jun 12  09:36:06 aiomysql                       {'ip': '68.98.11.83', 'user_agent': 'downlords-faf-client', 'player_id': 53471}
ERROR    Jun 12  09:36:06 LobbyConnection                Use async with instead
Traceback (most recent call last):
  File "/code/server/lobbyconnection.py", line 144, in on_message_received
    await handler(message)
  File "/code/server/lobbyconnection.py", line 526, in command_hello
    conforms_policy = await self.check_policy_conformity(player_id, message['unique_id'], self.session)
  File "/code/server/lobbyconnection.py", line 465, in check_policy_conformity
    with aiohttp.ClientSession(raise_for_status=True) as session:
  File "/usr/local/lib/python3.6/site-packages/aiohttp/client.py", line 883, in __enter__
    raise TypeError("Use async with instead")
TypeError: Use async with instead
WARNING  Jun 12  09:36:06 LobbyConnection                Aborting 68.98.11.83. Error processing command
ERROR    Jun 12  09:36:06 asyncio                        Unclosed client session
client_session: <aiohttp.client.ClientSession object at 0x7fe7e10386d8>

[Rackover]

Apart from that, it's working.