… users.

Previously during an error with invite_users_backend codepath, we returned
3 error items - invited email, error message, is_deactivated. However, for
organizations which haven't enabled EMAIL_ADDRESS_VISIBILITY_EVERYONE the
actual user email stored in the frontend is anonymised and different from
the invited email.

To fix this, we return back the anonymous emails for existing accounts along
with the other parameters. This results in 4 return values ->
    - email (invited email)
    - error_msg
    - is_deactivated
    - maybe_anonymous_email

Incase EMAIL_ADDRESS_VISIBILITY_EVERYONE is enabled for the realm,
maybe_anonymous_email would be same as the invited email. This is also
the case for cross_realm_bots. For invalid emails, we just return back
no strings.

Tests amended.

When the email_address_visibility realm setting is switched to
Realm.EMAIL_ADDRESS_VISIBILITY_EVERYONE, get_existing_user_errors
returns the maybe_anonymous_email parameter, which in this case
should match with the delivery_email.

This flag helps the frontend to know when to display the option of
subscribing users on invitation. Currently we show the option in two
cases:
   - When no one was invited (Probably all users exist already)
   - When some were invited while some weren't.

We do not show the subcription option when:
   - We fail to send any invites because emails were invalid.
   - Account is too new to send invites.

When inviting existing accounts in the realm, the user is presented
with an option to subscribe existing users to the marked streams.
The error option displays which accounts are being subscribed
to which streams.

The actual stream subscription can result in three cases:
    1. All the users were newly subscribed to the selected streams.
    2. All the users were already part of selected streams.
    3. Some users got newly subscribed, while some others were
       already subscribed.
Appropriate error messages are show.

Fixes zulip#15784.