Add UI and functions that enable users to adjust task quota

[orange0629]

Codes for issue #90

[orange0629]

If you want to try this new feature, add "allow_user_adjust_quota": True under "automatic_assignment" in your yaml project config file. The yaml should look as follows:

{
......
"automatic_assignment": {
      "on": True,
      "sampling_strategy": 'random',
      ......
      "allow_user_adjust_quota": True,   <- Newly added
    },
}

[orange0629]

Known issue: If an instance can be sampled multiple times, it's possible for a user to get a repeated instances after they increase their quota. Can anyone confirm if it's an expected behavior by design?