Bug 1023807 - [B2G][Calendar] Month view: Dot does not appears when even...

Added Dots for complete event dates
How about use `forEach` method to instead of the for loop? It looks more clear[1].
[1] https://github.com/mozilla-b2g/gaia/pull/20682/files#diff-fa69b42c7066e08948996ef0e2f0e977R102