chore(DTFS2-6892): commonise london timezone

[AlexBramhill]

Introduction ✏️

As part of sso, I will need to use the london iana timezone. It is not currently commonised.

Resolution ✔️

This ticket seeks to commonise the london timezone (outside of tests which are referring to it as a string), as well as creating a IanaTimezone type.

Miscellaneous ➕

• The IanaTimezone type is broad -- being a string. This is because calling the Intl.supportedValuesOf('timeZone') function returns a broad type (string). I do not want to overly specify this type by making a static list of iana timezones in this PR.
• I've destructured imports as per the existing convention
• I've tidied up mismatching require and import on some js files