fix button bg color on disabled combobox

* affinity group add/remove on instance settings tab

* say anti-affinity one fewer time

* Prevent adding instance to A-A group when not stopped

* extract disabledReason to its own function

* Only call it once

* Update test

* add instanceCan.addToAntiAffinityGroup

* tweak disabled message to match ones for stop/start/etc

* shorter disabledReason logic

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Co-authored-by: Charlie Park <charlie@oxidecomputer.com>

…#2790)

* Disable instance adding to anti-affinity group when instance is not stopped

* Use Message to explicitly note issue with non-stopped instances

* update test

* testing

* use new instanceCan

* change omicron source link for can add to affinity group

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Co-authored-by: David Crespo <david.crespo@oxidecomputer.com>

give silo and project access pages better titles