Support different time format preferences for the user

Many of the posts made by this plugin have a reference to a start time of an event or events. For any post we send directly to a user, we can form the time string in a way that this user prefers the time to be shown. The plugin can also be given a default value, to use for any public posts.

To determine a user's time display preference, we can use the plugin API to get the user's preferences, then find the one with Category: model.PreferenceCategoryDisplaySettings and Name: model.PreferenceNameUseMilitaryTime