Bug 951423 - [B2G][Calendar][Day View]Header displays truncated if date exceeds eighteen characters.

change L10n files to use abbreviated month name to avoid char cap. (eg: "Dec 31, Wednesday")

also added integration and unit tests to avoid regressions.

![proposal screenshot](https://bug951423.bugzilla.mozilla.org/attachment.cgi?id=8356691)

**notes:**

 - according to my tests the char limit was in fact 20 chars instead of 18.
 - `zh-TW` locale did not require any changes.
 - needs approval by the design team.

/cc @gaye