Allow global admins to edit user emails via the UI

[r4victor]

The UI should allow global admins to set/update user emails similar to how it allows editing user global role. Also, users should be able to see their emails – currently emails are only shown on Sky.

Manually assigning emails to users is useful for linking dstack users to OAuth accounts automatically (e.g. Okta, Entra).

The API already supports setting/updating emails.

[r4victor]

@olgenn, also ensure frontend passes all fields to /api/users/update. Currently, if you omit some fields (e.g. an email) – these fields will be updated to null. The API cannot differentiate between omitted and null fields (due to current pydantic limitations), so pass all.