Add short date options about cipherfox

[gavinhungry]

This is the nsIX509CertValidity object generated on my machine (Linux, en-US) from GitHub's certificate:

{
  "notBefore": 1457568000000000,
  "notBeforeLocalTime": "Wed 09 Mar 2016 04:00:00 PM PST",
  "notBeforeLocalDay": "03/09/2016",
  "notBeforeGMT": "Thu 10 Mar 2016 12:00:00 AM PST",
  "notAfter": 1526558400000000,
  "notAfterLocalTime": "Thu 17 May 2018 05:00:00 AM PST",
  "notAfterLocalDay": "05/17/2018",
  "notAfterGMT": "Thu 17 May 2018 12:00:00 PM PST"
}

CipherFox uses the notBeforeLocalDay and notAfterLocalDay keys to populate the $CERTISSUED and $CERTEXP formatting variables, respectively. Not all locales will present these in such a short format. This is the nsIX509CertValidity object generated by a Windows en-US machine from the same certificate:

{
  "notBefore": 1457568000000000,
  "notBeforeLocalTime": "Wednesday, March 9, 2016 4:00:00 PM",
  "notBeforeLocalDay": "Wednesday, March 9, 2016",
  "notBeforeGMT": "Thursday, March 10, 2016 12:00:00 AM",
  "notAfter": 1526558400000000,
  "notAfterLocalTime": "Thursday, May 17, 2018 5:00:00 AM",
  "notAfterLocalDay": "Thursday, May 17, 2018",
  "notAfterGMT": "Thursday, May 17, 2018 12:00:00 PM"
}

Instead, we can parse a new date object from the notBefore and notAfter timestamps (µs since the epoch).

[gavinhungry]

I think Date.prototype.toLocaleDateString may get us a consistently short, localized format:

var d = new Date(cert.validity.notBefore / 1000);

// all locales currently supported by CipherFox
[
  'de-DE', 'en-US', 'es-ES', 'fr-FR',
  'hu-HU', 'ja-JP', 'ko-KR', 'ru-RU',
  'zh-CN', 'zh-TW'
].forEach(function(locale) {
  console.log(locale, d.toLocaleDateString(locale));
});

de-DE  9.3.2016
en-US  3/9/2016
es-ES  9/3/2016
fr-FR  09/03/2016
hu-HU  2016. 03. 09.
ja-JP  2016/3/9
ko-KR  2016. 3. 9.
ru-RU  09.03.2016
zh-CN  2016/3/9
zh-TW  2016/3/9