Add an en-GB localization option.

This adds the Great Britain (or United Kingdom) locale.  Translation
isn't really needed in the `.po` file.  However, if someone wants to do
so it could be done.  They do have different spellings of some words.
The most important thing with this is that this adds the time format for
the Great Britain locale which is quite different from the US format.

Note that the `en.po` file was updated as well.  This file hasn't been
updated for a while since it really doesn't need to be.

htdocs/js/DatePicker/datepicker.js

@@ -5,6 +5,7 @@
 	const datetimeFormats = {
 		en: 'L/d/yy, h:mm a',
 		'en-US': 'L/d/yy, h:mm a',
+		'en-GB': 'dd/LL/yyyy, HH:mm',
 		'cs-CZ': 'dd.LL.yy H:mm',
 		de: 'dd.LL.yy, HH:mm',
 		el: 'd/L/yy, h:mm a',

htdocs/js/ProblemSetList/problemsetlist.js

@@ -158,6 +158,7 @@
 	const datetimeFormats = {
 		en: 'L/d/yy, h:mm a',
 		'en-US': 'L/d/yy, h:mm a',
+		'en-GB': 'dd/LL/yyyy, HH:mm',
 		'cs-CZ': 'dd.LL.yy H:mm',
 		de: 'dd.LL.yy, HH:mm',
 		el: 'd/L/yy, h:mm a',