Timezone improvements to `{add, edit}_downtime`

[gregcorbett]

Relates to #86

The Javascript elements on the downtime pages lead to some confusing behaviour when time offsets change (i.e. summer time to winter time).

In this case, the downtime should end at 30/10/2023 12:00 UTC, not 11:00 UTC - as the London/Europe timezone on the 30th is UTC. But when the downtime was entered, the London/Europe timezone was UTC-1 - hence 12:00.

• [ ] A relativity quick fix would be updating Site Timezone: Europe/London to something like Site Timezone: {UTC, UTC-1} (Europe/London) (or even just Site Timezone: {UTC, UTC-1} ) would make the behaviour clear - as it's more explicit that a fixed offset is being applied.
• [ ] A braver solution is reworking the timezone logic. We should look as to how feasible this is.

[gregcorbett]

A braver solution is reworking the timezone logic. We should look as to how feasible this is.

A good first step would be to create a toy example similar to the downtime page date/time picker that can handle StartTime and EndTime being in two different UTC offsets