Add rival field to BearSerializer

[Bekabyx]

A rival is a bear which has had multiple matchups with a given bear.

The criteria for this are:

• BearSerializer should have a field named rival
• rival will either be an object or null
• When a given bear has faced any other single bear on multiple occasions, rival will contain the data of the rival bear
  • Where a bear has multiple rivals, the opponent with which a bear has had the most matchups will be returned as that bear's rival
• When a given bear has only ever had unique pairings (i.e. has no rival), the rival field on BearSerializer will be set to null

"rival": null

"rival": {
    "url": "",  # String, URL to rival /bears/bear_uuid
    "bear_uuid": "",  # String, Rival bear's bear_uuid
    "matchups": 0,  # Integer, Number of times the given bear has been in a bracket with this bear
    "wins": 0 # Integer, Number of times the given bear has won a bracket against this bear
}