Remove login dialogs from browser history

[dbudzins]

Right now the sign in and sign up prompts have their own URLs. This is fine, but the URL's should get removed from the browser history when they close so that using the back button doesn't reopen them.

Repro 1:

• Login
• Use the back button and observe that the login prompt is shown again
  Expectations: The back button should go to whatever the screen prior to the login attempt was.

Repro 2

• Click login
• Click Cancel
• Use the back button and observe that the login prompt is displayed again
  Expectations: The back button should go to whatever the screen prior to opening the login prompt was

Probably just using replace instead of navigate will fix this. Please triage all dialogs and decide whether they make sense as part of the history or not.