This patch introduces HTMLTimezoneField, an HTMLForm field type that
allows the user to select a timezone, either from a geographic zone, by
manually entering an offset, or using the wiki/browser default. This
logic is extracted from DefaultPreferencesFactory so that it can be
reused elsewhere.
The widget itself is really just an HTMLSelectOrOtherField, it's just
the list of options and the JS logic that is special.
Bug: T309629
Change-Id: I99a00dff7e3319ce45883191daee16bec1ed68ba