Allow admins to edit email addresses of bot accounts

Summary:
e0a3e6ba3b87f96c83905e7188bb6161f7480784 explicitly mentioned
verified bots don't get a panel, but unverified bots are not a thing
(anymore) as the email address provided during creation is automatically
verified.

It's not outside the realm of possibilities that the email address might
need to change later on, so requiring manually editing the database
instead of using existing infrastructure is silly.

Closes T16111

Test Plan:
* As an admin, go to http://phorge.localhost/settings/user/SomeBotUser/, see new item "Email Addresses" in the panel on the left.
* As an admin, go to http://phorge.localhost/settings/user/SomeBotUser/page/email/ directly, don't get a 404 anymore.
* As a non-admin, go to http://phorge.localhost/settings/user/SomeBotUser/page/email/ and get a 404 as before.
* As an admin, on http://phorge.localhost/settings/user/SomeBotUser/page/email/ directly, add a new email address, mark it as primary, be able to remove the old email address. Check the phabricator_user.user_email DB table that the account's email address has been correctly updated.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16111

Differential Revision: https://we.phorge.it/D26090