Built-in Variable timestamp: support specifying a time zone

[damithc]

Current: the variable is always in GMT

Suggestion: give a way to output the timestamp in a specific timezone

Rationale: If the website is specific to a location e.g., our own module websites, it makes more sense to use the local time in the timestamp

[yash-chowdhary]

Would it be advisable to ask the user to set the variable in site.json like so -

{
"timeZone": "Australia/Brisbane"
}

We could also enable customization of the locale (using a "locale" key in the above file).

The other option for setting the locale would be that we fix it. The main options are en-US and en-GB.

E.g. en-US (12H system) would look like:
Sun, Mar 1, 2020, 9:02:49 PM GMT+10
Singapore time would thus be -
Sun, Mar 1, 2020, 7:01:08 PM GMT+8

vs en-GB (24H system) :
Sun, 1 Mar 2020, 21:03:05 GMT+10
Singapore time would thus be -
Sun, 1 Mar 2020, 19:01:28 GMT+8

I will proceed based on the feedback.

[crphang]

Would it be advisable to ask the user to set the variable in site.json like so -

Sounds alright to me. This approach is extensible, if there is a strong use case, we can still potentially explore overriding the "global" timezone defined in site.json. {{timestamp | timezone("your_timezone")}}.