Populate saved credentials efficiently to keep the user in context of the login

[sandysage]

When I want to log into an account with credentials I already have saved, I want to conveniently fill those in the web form so I can log in quickly.

Acceptance criteria

1. Should have control over what gets populated
2. Should be able to fill a web form in 3 clicks or less (reduces the current overhead of min of 5 clicks)
3. Should have a clear backup option for copy/paste for when fill doesn't work

User stories for:

• [ ] interaction design to enable user involvement (user control over what gets populated and where which won't happen automatically to start)
• [ ] collection of websites that we want to target to make sure it works
• [ ] port behind-the-scenes logic from Firefox for form detection
• [ ] move the data from Lockbox into the form
• [ ] wire it up in a way we can use it to fill usernames/passwords (implement the interaction design)

NOTE: This is the follow up to issue #241 progressing from making it easier to copy from the doorhanger to porting the existing login manager login into a Web Extension

Given the concerns recently highlighted where browser autofill can be abused for phishing attacks, we've decided to require user engagement to fill the data versus working towards complete auto-fill.

If we discover that users require more automation, we can build on this development (limited to no rework expected)

[sandysage]

This is the building block on top of #341 which uses the URI to provide more relevant results. Based on those more filtered results we can provide better tools for populating the login.