Bug 1027349 - [Calendar] month view week day names should be a single char r=gaye

created a different locale key instead of slicing the first char since I believe the slice wouldn't work well with all languages and would need custom JS logic to update the values every time the locale changed.
I would change the comment like this
```
# Used in month view as column header, each single char represents a week day
# (from 0-Sunday to 6-Saturday)
```