Use better variable names

Summary:
Use `p` and `c` instead of (reusing) `k`. This silences the following JSHint warning:

```
>>> Lint for webroot/rsrc/js/application/projects/behavior-project-boards.js:

   Warning  (W004) JSHintW004
    'k' is already defined.

              57       'phui-tag-shade-blue': (sum > 0 && !over_limit),
              58       'phui-tag-shade-red': (over_limit)
              59     };
    >>>       60     for (var k in color_map) {
              61       JX.DOM.alterClass(data.countTagNode, k, !!color_map[k]);
              62     }
              63   }
```

Test Plan: `arc lint`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11066