Otherwise channel groups don't show up correctly in MainUI (in actual
groups with a heading).

Note that this is completely backwards compatible - even though Things
change their ThingTypeUID after they're created, their UID does _not_
change, so no channel UIDs change either.

A happy consequence of building ChannelDefinitions for the ThingType is
that custom ChannelTypes are no longer necessary - the only variable parts
are labels and state/command descriptions, which are overridden by the
ThingType's ChannelDefinition and the DynamicStateDescriptionProvider
respectively. This can greatly increase responsiveness of MainUI if you
have many Homie things with many channels. My /rest/channel-types response
went from 2.6MB to 175KB. Since the full list of thing types is never
queried, there's no corresponding increase anywhere else.

Signed-off-by: Cody Cutrer <cody@cutrer.us>

Signed-off-by: Cody Cutrer <cody@cutrer.us>