Accept custom URLs for the Default User option

[joeyespo]

This was requested from an email.

By allowing the entire URL to be customized, this extension can then be used for Google Apps instead of just inbox.google.com. (Feel free to correct me on this.) Here's my work-in-progress design:

And if /u/ or https://inbox.google.com/u/ is in the value, it should probably be accepted and stripped out (instead of a save or runtime error). Bonus if it's some kind of editable dropdown with pre-filled users.

Side note: Later I think it'd be good to redesign the whole Default User option. Something that would lend itself to help with #3 and #35. But in the short-term, increasing option complexity to enable this feature isn't bad. Especially since it can help influence that new design.