Add email field to users table

[jaragunde]

User emails are required by the task creation reminder feature implemented in #334, which sends those reminders via email. That was implemented by appending a certain "@domain.com" to the user login name but, of course, actual email accounts could be different, and we should support that scenario.

Add the email field in the DB, the UI to edit it, and the means for #334 to use it (also use it in #391 and #429 if they are ever implemented).