Email sender in email alert callback should be set by transport_email_from_email

Hi,

When creating an email alert callback, the sender field is filled by default with "graylog@example.org" even if `transport_email_from_email` is set.

I think it's an error and that this field should be set to `transport_email_from_email` from `graylog.conf` if defined.
Only if `transport_email_from_email` is not defined, the default email should be "graylog@example.org".

## Expected Behavior

The email field should be set to `transport_email_from_email` value or "graylog@example.org"

## Current Behavior

The email field is set to "graylog@example.org". We have to delete the content of email send field and validate the form to get the email set to `transport_email_from_email`.

## Possible Solution

Here, the email is set to `graylog@example.org` in any case:
https://github.com/Graylog2/graylog2-server/blob/20ab7325eb82ec09c08ece0433e183412f32a05d/graylog2-server/src/main/java/org/graylog2/alarmcallbacks/EmailAlarmCallback.java#L177

I think it should be `emailConfiguration.getFromEmail()` and if `transport_email_from_email` is not set, `graylog@example.org`.

Adrien

* Graylog Version: 2.4.6