fix(users): Improve 'create user' cancelation process

Cancel and Save buttons will be shown only when form data is dirty
Preview pane is now toggling
By clicking 'Cancel' button when creating user, preview pane will close

SD-212
fix(http): improve http.timeout to catch options requests

SD-75 close #207
fix(users): fixed reloading list when user being edited is deleted.

SD-205: deleted user does not disappear from users list, unless page is reloaded
test(users): Tests for user editing.