Move Select box to left and style it as a checkbox

Summary: also fixes a SelectableListBehavior bug, fixes T936

Test Plan: Try checking/unchecking the users in `/settings/users` and performing actions on them individually and via the bulk dropdown. Try the bulk select/unselect all dropdown option.

Reviewers: rjmackay, vladimir, shadowhand

Reviewed By: shadowhand

Subscribers: srutto

Maniphest Tasks: T936

Differential Revision: https://phabricator.ushahidi.com/D452