Admin deactivating volunteer: choose whether email notification is sent immediately or not

[compwron]

Part of epic #4

What type of user is this for? [admin/volunteer/supervisor/all]
Admin

Where does/should this occur?
In admin dashboard under "Editing Volunteer"

Description
When an Admin changes a volunteer's status from "active" to "inactive," a dialogue box (future work not in this issue) should pop up, where one of the things that displays is a check box where Admins can indicate whether or not a deactivation email should be triggered immediately or sent later.
If Admin chooses "send later," volunteer deactivation should not trigger a deactivation email to volunteer. Instead, an Admin should be able to return to this same page to trigger that email notification at a later time.

Relates to issues: #228, #22

[compwron]

chatted about this 27 May 2020, we don't need this because the email text is pleasant