Add explicit ordering for help categories

[ArtOfCode-]

electrical:293267

The help center is grouped by post type to split out help/policy, then by help category, which is a custom field set by moderators. When displaying posts in the help center, there is no explicit ordering of categories set, which leads to unpredictable and sometimes unhelpful ordering.

Just sorting by category name alphabetically isn't likely to be very helpful - ideally we need some way to control which category appears first, regardless of name.