Add entitlement to LockedTokenManager

[joshuahannan]

Adds entitlements to LockedTokens.LockedTokenManager to prevent accessing privileged methods via reference downcasting

Need to make sure that these will still be valid with the migrations

[joshuahannan]

The only entitlement we've added for LockedTokens.LockedTokenManager is the UnlockTokens entitlement and we use that everywhere there is a LockedTokenMananger capability, so that should mean that the migrations will all work because the capabilities will all be migrated to their fully entitled types.

Additionally, I changed a lot of the fields in LockedTokenManager to be access(account). This is fine because none of those are accessed in transactions, only in the LockedTokens and FlowStakingCollection contract

[sisyphusSmiling]

Approved for merge to #319, only concern is the migrated type as you called out