Add a filter for the users fieldtype

[jacksleight]

This PR adds a filter for the users fieldtype:

It may not be the best approach, but I've used a select field rather than a users field in the filter form because:

1. That means you can add the "Me" option
2. The users field doesn't seem to work properly in the filter form, when you select a value the ID is displayed instead of the name

Guess this would be a problem if you had a lot of users. Is there a better way?

Related https://github.com/statamic/cms/issues/4101 (doesn't include searching the user fields)