Allow multiple accounts for a single instance

[chucker]

Requested at https://mstdn.social/@MaryHenryGEO/110944566802463482

@IBAction func next(_ sender: Any?) ultimately finds an existing Keychain entry with func clientAppRegistration(for instanceBaseUrl: String) -> ClientApplication?. At no point is there a prompt if you want an additional account.

Semi-related to #89 and #118.

[chucker]

So, maybe

• if you don't have an account with that instance yet

(•) Sign up or log in at MyInstance.social
( ) View MyInstance.social as a guest

[ Next ]

• if you do

You already have an account with MyInstance.social.

(•) Add an additional account at MyInstance.social
( ) View MyInstance.social as a guest

(Does it make sense to add an instance as a guest if you also have a registered account? Hmm.)