Added timezone-based datetime to comments

Added timezone-based datetime to comments using Intl.DateTimeFormat(), so that users can better orient to the chronological flow of the conversation (like how we read the time stamps off our chat apps sometimes). Thanks to @Lobacrow for the insight!

formatDatetime: (dateStr, string) =>
Intl.DateTimeFormat('en-GB', {
year: 'numeric',
month: 'short',
day: 'numeric',
hour: '2-digit',
minute: 'numeric',
hourCycle: 'h12',
timeZone: Intl.DateTimeFormat().resolvedOptions().timeZone,
}).format(new Date(dateStr)),

(Dev notes: This also sets me up to deal with the timezone-based streak calculation, one step fwd at a time)