Add 'Save user(s)' action

Description of the need

When some code is run in hook_user_presave(), an easy way to bulk update user accounts would be to use the actions at the top of /admin/people/list. However there's not currently an action to save the user account (to trigger this hook).

Proposed solution

Add an action that simply saves the user account(s), triggering the presave hook.

Alternatives that have been considered

An alternate solution is to use the 'Block user(s)' action. That triggers the presave hook, but then you need to unblock the users again. That triggers (by default) an email to be sent to users, and could be problematic if you'd prefer to have these changes made without triggering emails to be sent...

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes a 'Save user(s)' action.