[VoiceOver] Add Self-hosted Site page concerns

[shiki]

Main

• The site address can have an accessibilityLabel
• Low hanging fruit: The “Enter the address“ label has been updated in Change copy per Editorial and to reflect Android app WordPressAuthenticator-iOS#123 but seems to be incomplete to fully close Update error message when entering an invalid site address #12188.
• The error should be announced

Credentials

• 1Password button is not read right
• The show password is button has no label
• The Username and password fields can have labels
• “Lost your password“ opens Safari. It should use the link attribute instead of button.

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Tested on iPhone XS, iOS 13.1.3, WPiOS develop (0d869c5)