Manage multiple identities of the same type in User Settings page

## Goal

Formally support multiple identities, allow end-user to self-manage their more than one email/phone/username in a given account using the User Settings page

## Solution

Affordance in the `/settings` page

* Add/Edit/Remove Email Addresses
* Add/Edit/Remove Phone Numbers
* Add/Edit/Remove Usernames
* Select primary email address
* Select primary phone number
* Select primary username

Notes:

* These actions can be disabled
* e.g. it can be configured such that a user cannot remove their email address but can edit it.
* The admin can set minimum and maximum identities of a user.
* For example, if every user must have minimum 1 and maximum 3 email addresses:
* The end-user cannot remove their last email address
* The end-user cannot add more email address if there are already 3 in their account

Start by tracing the existing /settings page and current email, phone, and username identity flows. Map the add, edit, remove, and primary-selection behavior for each identity type, including configurable actions and minimum or maximum limits. Done means these settings are consistently supported for all three identity types and their constraints are enforced.

Written by the indexing model from the issue text.