[Hockey] Update for new API. (#328)

* [Hockey] Reorganize core loops to utilize new API methods.
Start building around the new API.

* Get previews working.
Fix goals duplicate posting.
Make pickems, gdt, and gdc work on the new API system.
Restructure Schedule to more easily list specific days of games.

* Use GameType Enum properly.
Add header to requests.
Close session when cog is unloaded.

* Update Standings for new API.
Make schedule and games command work. These might require more work in the future.
Allow api schedule commands to specify a date.

hockey/abc.py

@@ -8,6 +8,7 @@
 from redbot.core import Config, commands
 from redbot.core.bot import Red
 
+from .api import HockeyAPI
 from .game import Game
 from .helper import (
     DateFinder,
@@ -39,6 +40,7 @@ def __init__(self, *_args):
         self.session: aiohttp.ClientSession
         self.pickems_config: Config
         self._ready: asyncio.Event
+        self.api: HockeyAPI
 
     #######################################################################
     # hockey_commands.py                                                  #