Define wp_is_mobile() function

It's normally defined in `vars.php`, and some plugins / themes expect it
Merge pull request #1546 from wp-cli/1542-define-wp-is-mobile

Define wp_is_mobile() function
Introduce `wp user list --network` shorthand

This makes it easier to list all users in a network.
Roles shouldn't be exposed with `--network`
Test for #1548
Use `db_id` as the id for `wp menu list item`
Merge pull request #1549 from wp-cli/1344-list-network-users

Introduce `wp user list --network` shorthand
Merge pull request #1550 from wp-cli/fix-1548

Use `db_id` as the id for `wp menu list item`