Project stats data model updates

[heyheyjp]

Replaces #368, to be merged into feature integration branch features/stats instead of directly into master.

Fixes #365.

Objectives:

• Group cumulative stats in a single object (we'll be tracking more than ECC now)
• Make explicit the meaning of property values (also more consistent with other models)
• Orient player stats in a project-centric way instead of a cycle-centric way

Main Change:

Player stats data previously stored as...

{
  "ecc": 311,
  "cycleProjectECC": {
    "204125f3-ece7-42bb-8a86-a619682ff379": {
      "48382d94-dcd7-479a-9229-7db4bb364916": {
        "abc": 3,
        "ecc": 108,
        "rc": 36,
      },
    } ,
  },
}

...is now stored as...

{
  "stats": {
    "ecc": 311,

    "projects": {
      "48382d94-dcd7-479a-9229-7db4bb364916": {
        "cycles": {
          "204125f3-ece7-42bb-8a86-a619682ff379": {
            "abc": 3,
            "ecc": 108,
            "rc": 36,
          },
        },
      },
    },
  },
}

Other Changes:

• Renamed stats-related functions referencing ECC specifically to more generic stats-related names