0.19 account reauth

[EricBAndrews]

Checklist

• I have read CONTRIBUTING.md
• I have described what this PR contains
• This PR addresses one or more open issues that were assigned to me:
  - Slack
• If this PR alters the UI, I have attached pictures/videos

Pull Request Information

This PR adds some logic to prompt users to reauthenticate if their saved credentials are invalid, namely:

• invalidSession errors are now thrown for any credential failure response
  • AddSavedInstanceView now catches those as login failures instead of manually checking for .isIncorrectLogin on a response error
• The expired session notification sheet is now attached normally to ContentView instead of being programmatically presented through the NotificationDisplayer
• The expired session notification sheet now resets the invalidSession flag on the ErrorHandler when dismissed, which prevents an invalid session message being erroneously displayed when cancelling the token refresh and switching accounts

Testing steps

To simulate the use case of users attempting to use accounts saved in Mlem before their instance updated to 0.19, perform the following:

• Run a local Lemmy instance on a 0.18 stable release build
• Create some accounts and add them to Mlem
• Update the local instance to 0.19
• Relaunch Mlem

The new flow should trigger.

For thoroughness, I have tested both the cases where the accounts were added in an older version of Mlem (release-1.1) and the latest TestFlight build (beta-1.1.1-RC-2), though that shouldn't matter.

[Sjmarf]

Looks good! Does this close #791?

[EricBAndrews]

It does! Nice spot