Add some sort of notice to view pages of plans that have been deprecated.

[nellshamrell]

Habitat core plans recently adopted a plan deprecation strategy

One of the relevant points required some Builder work:

Add some sort of indication to the core plan's view page in Builder (remember, even if it is set to private and not viewable publicly, it will still be viewable to members of the core origin) (this will likely require some feature work)

When a plan is deprecated, it would be quite useful to somehow indicate this on the Builder page for that plan (we will also be indicating it in the README of the plan in the core plans repo).

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