Allow spaces to be used in mentions

Problem

On platforms with a lot of users there can be many people with the same first and last name. There can also be users with usernames that match short first names. When mentioning a user only 10 people are shown. These factors combined could cause it to be hard or impossible for a user to mention someone without knowing their full username (which may not be displayed anywhere).

Solution

Allow mention selection to carry on past a space, this would allow to select "John Zorro" when there are already many John's on the platform. This also brings the mentioning functionality inline with how you would expect this to work on other social platforms.