Recognize and do the right thing with more English locales

Previously, our date-parsing code knew about only four English locales, which it had hard-coded: en-US (United States), en-GB (United Kingdom), en-AU (Australia), and en-CA (Canada). Now, rather than hard-coding a list of those four specific English locales, when the user's OS is using an English locale we query the operating system to determine how dates are formatted in that locale and configure the date-parsing code dynamically to match the operating system. This means that Send Later should correctly format and parse any English locale that the operating system knows about.