Handle Collator Candidate to Invulnerable Path

[joepetrowski]

When registering as a candidate, Collator Selection ensures that the user is not already invulnerable. However, it is possible that a candidate becomes invulnerable via governance referendum (either within set_invulnerables or add_invulnerable).

When adding an invulnerable, the system should check if they are a candidate, and try to remove them (it may not be possible due to T::MinCandidates, but it should try).

Edit: On further thought, we should reconsider this MinCandidates item. It does not include Invulnerables, but it's there AFAIK to make sure there is always one collator in the set. Invulnerables should fulfill this requirement, so I think that MinCandidates should include Invulnerables and be renamed to MinSetSize.