UNKNOWN and UNDECIDED use different enum values but are not permitted in the same context

Original report
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Some bugs that are using launchpad for bug tracking, have an importance of UNKNOWN. This importance shouldn't be used by users, it is reserved for bug watches that can't determine a remote bug's importance. We shoud convert all existing bugs to UNDECIDED.

Analysis
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There are a cluster of defects (all marked as duplicates of this bug) that have to do with UNKNOWN and UNDECIDED.

It is arguable that remote trackers might have a native UNDECIDED that we would reflect as UNDECIDED into LP, but lack-of-knowledge in a mirrored bug is orthogonal to any given field (because either our mirror status is up to date, and we know, or its stale, and we don't know anything at all). In the absence of remote+UNDECIDED, UNKNOWN and UNDECIDED become mutually exclusive:
 - contexts that do not 'use LP' are only permitted bug watches
 - context that 'use LP' are not permitted bug watches, and are not permitted UNKNOWN.

We can address all these issues by consolidating the UNDECIDED and UNKNOWN enums and tracking 'mirrored metadata is out of date' as a wholly separate flag on bugwatches.

Our current model is more than a little misleading (when do we ever truely know a remote status :P)