Feature/game structure tweaks

- Reversed the linkage between Tournament and Game relationship. Fixed all the issues related to this change.
- Added "Small Game Thumbnail" to Game model.
- Allowed Game model to have more than one Channel Name. To achieve this I added a new table called 'Channel' and added a one-to-many relationship between Channel and Game models. Each channel has a 'primary' boolean attribute. 
- Added "Header image glow hex color" to Game model. Added ColourField custom django field class to handle the validation of this field.

#53