Bugzilla

Design - 
Should allow the user to:
- From a list of multiple usernames, select one
- From a list of passwords, select one
- Disttinguish between updating a saved login & saving a new one
- Easily distinguish between logins populated from storage (already saved) and values pulled from input fields (things we think they might want to save)
- Allow a user to select from multiple passwords without surprising the user by displaying unmasked passwords
- By default, should display the UN/PW that we currently show

Should handle the following edge cases:
- No logins saved for the current page (e.g., if there is a dropdown icon, hide it)
- Too many saved logins to display without scrolling

Technical notes -
The password save/update doorhanger is defined in `popup-notifications.inc`.  Many normal HTML elements cannot be used in this file, however <input is="autocomplete-input"/> works as expected.

Create a UI capable of displaying usernames according to the spec at https://mozilla.invisionapp.com/share/XGXL6WZVKFJ#/420547613_Updated_State-_June_2020.  UI for passwords will be added in a later story.

Note that there will be no functional changes in this bug.  Later bugs will hook this UI up to data, changing the display.