Improve Account, SignIn, SignOut & ISStoreAccount extension & associated code:

• Migrate ISStoreAccount from Grand Central Dispatch to PromiseKit & improve PromiseKit usage
• Prevent ISAccountService.signIn(with:) deprecation warning
• Improve macOS version handling & other encapsulation
• Improve "Already Signed In" error output
• Output error on blank password
• Remove unnecessary output
• Simplify & clean code