Validate specified timezone against IANA database

[jmooring]

Case 1: Site Configuration

If an invalid timeZone is specified in the site configuration, the failure is silent. Example:

timeZone = "America/LosAngeles"   # Should be America/Los_Angeles

Case 2: Time Function

If an invalid timezone is specified as the optional second argument to the time function, the failure is also silent (the build continues without error or warning). However, the error is rendered on the page (easy to miss and then inadvertently deploy). Example:

{{ time "2020-07-31T09:29:26" "America/LosAngeles" }}

This renders:

unknown time zone America/LosAngeles

[bep]

The second case above is #8835 -- I separated that into its own issue, because that is more general and technical. We do return an error on invalid timezone in time.AsTime, but due to some technical mumbo jumbo, that error does not transform into a real error when you use the short form time.