WIP: Toogle new mail receiver

Closes no issue

What does this MR do?

The input field for an additional e-mail address is hidden and shown again with a plus button.

Reason: Because mails with larger content the sender slips up and can no longer be changed.

How confident are you it won't break things if deployed?

(be honest!)

Links to related issues

Thread on #foodsharing-beta: https://yunity.slack.com/archives/C74JC2V26/p1580340709045900
Thread on #foodsharing-dev: https://yunity.slack.com/archives/C1T1ZM9A4/p1580580790212900

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Import sql INSERT INTO `fs_mailbox_message` (`id`, `mailbox_id`, `folder`, `sender`, `to`, `subject`, `body`, `body_html`, `time`, `attach`, `read`, `answer`) VALUES (102410, 19708, 2, '{\"host\":\"foodsharing.network\",\"mailbox\":\"region-341\",\"personal\":\"Foodsharing Anmeldevorgang Quiz\"}', '[{\"personal\":\"b.menzel@foodsharing.network\",\"mailbox\":\"b.menzel\",\"host\":\"foodsharing.network\"}]', 'selfmail1', 'selfmail1-1', 'selfmail1-1', '2019-10-16 21:58:46', '', 1, 0);
3. Login as store manager or amb (bot)
4. Than is the mail visible on dashboard. The message can be open only over the dashboard.
5. Click the button "Antworten" in the message
6. Insert a long text until scrolling is available.
7. Is the mail sender showing?

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings