Authentication providers - Account management UX

[RMacfarlane]

Now that extensions can register authentication providers, we are able to display information about what accounts a user is currently signed into. Instead of having extensions display this in the status bar, we can centralize this elsewhere, potentially in the activity bar above the gear icon.

Users should be able to:

• View current accounts
• View account status (For example, if offline and currently unable to access the account)
• Sign out of an account
• Sign into another account for a particular provider