Allow timezone to be specified for Single Daily Build or Cron triggers

When specifying the time in a "Single Daily Build" or cron-based build trigger, it's unclear what timezone this is in. This is a problem when, for example, your bamboo server is in one timezone, your build agents are in another timezone, and your workstation in yet another.

At the very least, the UI should tell the user which timezone will be applied, but better yet the UI should allow the user to specify the timezone explicitly, to avoid ambiguity and to make it clear in the plan config that a given build is to run at e.g. 7pm Sydney time, regardless of where the various components of the build system are sitting.